Methods and apparatus for producing ferrate(vi)

ABSTRACT

An undivided electrochemical cell. The electrochemical cell includes a housing defining an undivided chamber, the housing having one electrolyte inlet and at least two electrolyte outlets; an anode in the chamber; a cathode in the chamber; and an electrolyte in the chamber, wherein the anode and the cathode are not gas diffusion electrodes. The invention also involves a method of operating an electrochemical cell, and methods for making ferrate(VI).

The present invention relates generally to electrochemical cells andmore particularly to undivided electrochemical cells, methods ofoperating undivided electrochemical cells, and methods for makingferrate(VI) and for making certain ferrate(VI) products.

Ferrate(VI) is a strong oxidizer and produces a water impurity coagulantand precipitant in water. These properties make ferrate(VI) useful forwater decontamination and purification such as industrial waste waters,farming process waters, sewage treatment plants, and in the productionof potable water supplies. It is also useful as battery materials, inchemicals production, for metal surface corrosion control, surfacedecontamination and cleaning and many other industrial applications.

The two basic methods of making ferrate(VI) in aqueous solution arechemical and electrochemical. To date, only chlorination chemicalmethods have shown utility in preparing isolated useful solid forms offerrate(VI), producing only a few grams at a time. Chemical methodsinvolve contacting a ferric iron compound with an oxidizing material,usually hypochlorite, in an aqueous alkaline environment (wet route), orat high temperature (dry route). However, the use of hypochlorites isundesirable because of difficulty in operation, nonscaleability of theprocess, contamination of the product, large waste streams, productionof chlorine gas byproducts and expensive raw materials.

Electrochemical methods for producing reactive products or separationshave typically utilized divided cells. In divided cells, an ion-transferpolymer's or ceramic frit membrane separates the liquids in the cellinto anode and cathode chambers. For ferrate(VI) solution preparation,there is a sacrificial anode made of an iron containing material. Thecathode can be made of various materials including iron, carbon, nickel,carbon steel, stainless steel, nickel plated iron, or combinationsthereof. Concentrated aqueous sodium hydroxide is normally introducedinto the bottom of the anode chamber and removed from the top.Similarly, sodium hydroxide is introduced into the bottom of the cathodechamber and removed from the top. Electrical current, or just “current”is applied across the cell causing the anode to oxidize to water solubleferrate(VI), which is carried off with the anolyte.

Prior work on electrochemical cells suggested that undivided cells wereundesirable for making ferrate(VI) because the electrolytic efficiencydropped off rapidly with operation time, the ferrate(VI) is decomposed(chemically reduced) at the cathode, the ferrate so produced, decomposedquickly and only dilute ferrate solutions could be prepared. A recentpublished application, US 2003/0159942 A1 (Minevski), describes a singlechamber electrochemical cell that is said to be useful for theproduction of ferrate salts for short periods. The cell has oneelectrolyte inlet and one electrolyte outlet on opposite ends of thechamber. The electrolyte is an aqueous hydroxide solution comprising oneor more alkali metal hydroxides, one or more alkaline earth metalhydroxides, or combinations. The hydroxide concentration is betweenabout 1 molar and about 30 molar. The molar ratio of KOH to NaOH is upto about 5, and is preferably between about 1 and about 3. The cell caninclude an optional porous frit between the anode and cathode. However,porous frits are brittle and subject to dissolving in caustic, cracking,pluggage with iron oxide solids, and are thick which raises operationalvoltages, thereby increasing heat, power consumption, slowing productionrates, and increasing cost.

Minevski describes a process that includes continuous filtration usingmethods including magnetic means. It describes the “ferrate” as beingattracted to a magnetic surface, i.e., a ferromagnetic material.However, ferrate(VI) is a paramagnetic material (containing unpairedelectrons) but not a ferromagnetic material. Paramagnetic materials arenot sufficiently attracted to magnetic surfaces to allow simple magneticseparations. Ferromagnetic particles (aligned magnetic moments) arerequired for such separation and they exhibit an external orderedmagnetic field that is attracted to or repulsed by external magneticsdepending on relative direction of magnetic field alignment. Ferrate(VI)does not have ferromagnetic particles and is not attracted to magneticsurfaces. However, ferrate(VI) can contain loose ferromagneticimpurities (such as magnetite Fe₃O₄) which are magnetic but are notstrong oxidants or sufficiently water soluble, and are therefore of novalue to the uses of ferrate(VI). Hrostowski et al., “The MagneticSusceptibility of Potassium Ferrate,” Journal of Chemical Physics, Vol.18, No. 1, 105-107, 1950; Shinjo et al., Internal Magnetic Field at Fein Hexavalent States, J. Phys. Soc. Japan 26 (1969) 1547; Oosterhuis etal., “Paramagnetic Hyperfine Interactions in an e_(s) ² Configuration ofFe,” Journal of Chemical Physics, Vol. 57, No. 10, 4304-4307, 1972; andHoy et al., “Critical Slowing Down of Spin Fluctuations in K₂FeO₄,”Journal of Magnetism and Magnetic Materials 15-18 (1980) 627-628, areincorporated herein by reference for the description of the paramagneticcharacter of ferrate(VI) and the existence of ferromagnetic impuritiesin ferrate(VI). Note that these magnetic impurities are as particlesthat are physically separate from the ferrate(VI) crystals, and these donot agglomerate together and hence do not offer a means to recover bothferrate(VI) and the magnetic particles together magnetically. Minevski'sdescription of magnetic separation suggests that its process primarilyproduced magnetic iron compounds, such as magnetites rather thanferrate(VI) compounds. The authors of the invention described herein forferrate(VI) compounds are also aware of the conditions for operating theelectrochemical cell to produce magnetic iron oxide products of the typedescribed by Minevski, and these inventors also have invented means toavoid such unwanted byproducts, and these means are described herein.

The prior art teaches that while electrochemical processes may be usefulfor laboratory scale production of ferrate(VI), they are unsuitable forcommercial scale production for several reasons: 1). First, they canonly be run for short periods of time (a few hours) before the cell mustbe shut down and cleaned 2) During ferrate(VI) production, some Fe(VI)degrades to Fe(III), which is insoluble in hydroxide solutions. 3) TheFe(III) precipitates out of solution, coating the cell walls and, individed cells, plugs the pores of the membrane as well. 4) This foulingresults in decreased current efficiency and ferrate production. In suchinstances, the ferrate production will decrease until it is less thanferrate decomposition. Moreover, the use of membranes in divided cellsincreases the cost dramatically for materials, labor, and more thantriples the required electrical power.

As a result of these problems, there is currently no regular commercialsupply of even small laboratory quantities of compounds of ferrate(VI)using any synthetic or production process method.

Therefore, there is a need for a commercially feasible method andapparatus for making ferrate(VI), and for an electrochemical cell whichis simple, inexpensive, forms insignificant amount of by-products, andis easy to operate, even at a large scale of production.

The present invention meets this need by providing an undividedelectrochemical cell which is designed and operated in a manner whichavoids by-products, is highly energy efficient, scaleable, and operatescontinuously. The undivided electrochemical cell includes a housingdefining an undivided chamber, the housing having one electrolyte inletand at least one outlet, one located to gather electrolyte from thecathode side and one which gathers electrolyte from the anode side; ananode in the chamber; a cathode in the chamber; and an electrolyte inthe chamber, wherein the anode and the cathode are not gas diffusionelectrodes. The invention includes one, or preferably a “stack”, of suchcells.

The electrochemical cell of the invention preferentially includes afluid controller in fluid communication with the electrolyte outlets.Suitable fluid controllers include, but are not limited to flowrestrictions, valves, bends in fluid flow direction, weirs havingdifferent heights, or constrictions in one or more of the exit lines.

The electrochemical cell of the invention most preferably includes ascreen between the anode and the cathode. It cannot include a membrane.Membranes are cell inserts which physically separates the fluid aroundthe cathode from the fluid around the anode, and adds significantly tothe voltage drop across the cell when compared with the same celldimensions and design without the membrane, for example by severaltenths of a volt, and even several volts, where the electrochemictryonly requires at most a few (<4) volts. On the other hand, screensprovide an undivided cell in that they allow intermixing of anolyte andcatholyte and do not show this costly voltage drop. Screens also allowthe gassing and fluid mixing hydrodynamics to be different on each sideof the screen.

Another aspect of the invention is a method of operating an undividedelectrochemical cell. The method includes providing a housing definingan undivided chamber, the housing having at least one electrolyte inlet,and preferably at least two outlets, an anode in the chamber, and acathode in the chamber; introducing an electrolyte into the chamberthrough the electrolyte inlet; and controlling an amount of electrolyteand/or gas flowing out of the outlets so that substantially moreelectrolyte flows past one electrode than the other. Preferably, thechamber also contains the above-mentioned screen. Most preferred is thatthe chamber also contains the screen and exit fluid controller.

Another aspect of the invention is a method for making ferrate(VI). Themethod includes providing an undivided electrochemical cell comprisingan iron-containing anode, or an inert anode with an iron containingelectrolyte particulate slurry, a cathode, and an electrolyte solution,the electrolyte solution comprising an aqueous solution of NaOH, or amixture of KOH and NaOH, wherein a molar concentration of NaOH isgreater than about 5 and a molar ratio of KOH to NaOH of less than 0.4,preferably less than 0.25, and most preferably less than 0.12; andapplying a voltage between the anode and the cathode to form theferrate(VI) solutions and compounds.

Still another aspect of the invention is a method for making ferrate(VI)which includes providing an electrochemical cell comprising aniron-containing anode, a cathode, and an electrolyte solution, theelectrolyte solution comprising at least one hydroxide; and applying avariable direct current voltage between the anode and the cathode toform the ferrate(VI), the variable direct current voltage varyingbetween a maximum voltage (Vmax) and a minimum voltage (Vmin), theminimum voltage being greater than 0, and the maximum voltage being inthe range 0.7-4.0 volts, with current densities varying between Vmax andVmin in the range of 0.1-200 mA/cm².

Another aspect of the invention is a method for making ferrate(VI) whichincludes providing a housing defining an undivided chamber, the housinghaving an electrolyte inlet, at least one electrolyte outlets, aniron-containing anode in the chamber, and a cathode in the chamber;introducing an electrolyte solution into the chamber through theelectrolyte inlet, the electrolyte solution comprising at least NaOH,wherein a molar concentration of NaOH is greater than about 5; flowingelectrolyte out the of outlet; applying a variable DC voltage betweenthe anode and the cathode of sufficient amplitude to form theferrate(VI), the variable direct current voltage varying between amaximum voltage and a minimum voltage, the minimum applied voltage,being 0 or greater. Typically the method may include the step ofapplying the variable DC voltage obtain a voltage level where ferrateactive film removal exceeds or equals net active film formation rate fora selected time period, said time period selected to substantiallyprevent excessive film growth. In other aspect of the invention theminimum applied voltage is greater than 0.

In another aspect of the invention there is disclosed an apparatus foran undivided electrochemical cell made up of a housing defining anundivided chamber, the housing having one electrolyte inlet and at leasttwo outlets; an anode in the chamber; a cathode in the chamber; and anelectrolyte in the chamber, wherein the anode and the cathode are notgas diffusion electrodes.

Still another aspect of this invention is the provision of a new,electrochemically active oxide of iron.

FIG. 1A is a representative waveform for ferrate(VI) production. Appliedwaveform is a square wave at about 1 Hz with Vmax seet at 320 msec. andVmin adjusted to 80 msec. using power control circuit of FIG. 2. Atthese cell conditions, Vmax is 2.32 V and Vmin is 1.2 V. Illustratesrelationship between power signal control parameters and electrochemicalreactions.

FIG. 1B is an oscilloscope meter display of waveforms according to thepresent invention.

FIG. 2 is a schematic diagram of a controller, actually used in theexamples herein, for controlling a power supply for providingappropriate varying DC for the invention.

FIG. 3 is a schematic diagram of a proposed version of a controller forcontrolling a power supply for providing appropriate varying DCaccording to the invention.

FIG. 4 is a schematic diagram of one version of a controller (that usesa microprocessor) for controlling a power supply for providingappropriate varying DC to the apparatus of the invention.

FIG. 5 is one embodiment of an electrochemical cell according to thepresent invention which illustrates electrolyte flow pattern around theelectrodes, and replenishment pattern of electrolyte through the screento the cathode.

FIGS. 6A and 6B are perspective views of electrolyte overflows usingweirs for an anode/spacer/cathode assembly.

FIG. 7 is a cutaway view of one embodiment of an electrochemical cellaccording to the invention showing a typical face of a hanginganode/cathode.

FIG. 8 is a cutaway side view of one embodiment of an electrochemicalcell showing a typical screen framed in the spacer. In some embodimentsthe screen is not present, only its support spacer.

FIG. 9 is a top view of a typical layout for anode/spacer/cathodecombinations. Spacer optionally holds a screen (not shown).

FIG. 10 is a face view of a typical anode/spacer/cathode arrangementshowing their relative sizes and positioning, according to one aspect ofthe invention.

FIG. 11 is a side view of a typical anode, cathode, and screen layoutaccording to yet another aspect of the invention. It illustrates an endview of anodes slightly shorter than cathodes to achieve betterelectrical field distribution.

FIG. 12A is a side view of a typical anode, cathode, and screen layoutaccording to another embodiment of the invention.

FIG. 12B illustrates a center cutaway view of the apparatus of FIG. 12A

FIG. 12C illustrates the catholyte exit cell end panel with ports forcatholyte over flow.

FIG. 12D illustrates the anolyte exit cell end panel with ports foranolyte over flow.

FIG. 13 is a side view of an “L” shaped flow deflector spacers.

FIG. 14A is a side view of the tank showing a typical electrode stackfor one embodiment.

FIG. 14B is a tip view of the bottom of the tank showing electrolytefeed to the tank.

FIG. 15 is schematic showing electrode side and button spacers.

FIG. 16 is a schematic diagram of a laboratory apparatus typical of theinvention.

FIG. 17 is a graph depicting the production of ferrate where the ferrate(VI) concentration is on the left vertical scale in mM and the time inminutes is on the horizontal scale. Open squares represent measurementsat the 785 nm peak and pone triangles represent measurements at the 505nm peak.

FIG. 18 is a graph showing the results from the production offerrate(VI). The weight of the potassium ferrate produced is shown onthe left vertical scale in grams; the time interval in minutes is shownon the horizontal scale. Ferrate (VI) was harvested at four intervalsshown on the graph starting at about) minutes, 1100 minutes, 2300minutes and 3900 minutes. The graph shows that harvesting greatlyincreased the rate of production of potassium ferrate. That is each timethe product was removed by filtration or centrifugation, the productionrate (increased slope of line) increased over that of the average slopethat is depicted by the long straight line.

FIG. 19 is a schematic drawing of one typical embodiment of theinvention having three cathodes and two anodes.

FIG. 20 is a schematic diagram of one embodiment of an overall apparatusfor production of ferrates.

FIG. 21 is a graph depicting ferrate (VI) concentration, and theproduction rate of ferrate(VI) versus time.

FIG. 22 is a graph depicting ferrate(VI) concentration, and theproduction rate of ferrate(VI) versus time.

FIG. 23 is a graph depicting the ferrate(VI) concentration, versus timefor a continuous production run.

FIG. 24 is a graph of ferrate(VI) visible absorption spectrum. The Yaxis is Absorbance in absorbance units and the X axis is Wavelength innanometers.

FIG. 25 is a graph of total iron UV/visible absorption spectrum. The Yaxis is Absorbance in absorbance units. And the X axis is Wavelength innanometers.

FIG. 26 shows a typical embodiment for a bipolar electrochemical cellarrangement.

Broadly, the invention provides for apparatus for producing oxometalions e.g., ferrate(VI)) using an undivided electrochemical cell.Typically, the electrochemical cell includes a housing defining anundivided chamber, the housing having at least one electrolyte inlet andat least one outlet for gas and/or electrolyte, or one gas and oneliquid electrolyte outlets; an anode in the chamber; a cathode in thechamber; and an electrolyte in the chamber, wherein the anode and thecathode are not gas diffusion electrodes. There is a power supply forgenerating a variable direct current and voltage for application acrossthe anode and cathode. The direct current voltage is typically appliedso as to have a peak voltage, Vmax, and a minimum voltage, Vmin. Vmin isabove 0 volts, and is that voltage required to substantially avoidpassivation of the anode surface (as described in the detaileddescription section).

Active Ferrate(VI)—Producing Oxide Film:

While not wishing to be bound by theory, it is believed that thefollowing anode surface iron oxide reaction mechanism applies tounderstanding the invention of achieving continuous and efficientelectrochemical production of ferrate(VI) compounds. Under conditions ofthe invention, an iron anode typically forms a uniformly red-orangecolored, smooth textured, non-flaking, non-crumbly, thin, “active” ironoxide surface layer as an intermediate in the formation of ferrate(VI).Formation and control of this active, unpassivated oxide surface layeris unexpected and is believed to be formed by the reaction of Fe(0) toform certain Fe_(X)O_(Y) “oxides of iron”. Passivating oxide films ofiron typically have formulas such as FeOOH, Fe₂O₃, Fe(OH)₃, and Fe₃O₄.Color of such oxides varies with particle and grain size, and/or degreeof hydration and wetness. The red-orange oxide film, indicative of anactive ferrate(VI) producing surface, appears to be a single orcombination of these oxides, or an entirely different formula. Thered-orange film is reactive as it only persists for a few hours onceisolated in room air, whereupon it changes to more conventionalyellow-orange, black, and brown colors. Such surface colors are alsoassociated with iron anodes which do not produce ferrate(VI). Anappropriate reactive composition for the red-orange oxide film of theinvention might reasonably contain a blend of hydrated Fe(III) andFe(IV) oxides. It is believed that the active film is not Fe(III) oxidesalone, especially Fe₂O₃, as such oxides are kinetically very inert andso slow to react (passivating), and not expected to make an effectivereactive intermediate for efficient ferrate(VI) preparation. In fact,the red-orange active oxide film may be primarily Fe(IV)-based [e.g.hydrated Fe^(VI)O(OH)₂, or the equivalent], thereby by-passingwell-known and sluggish reacting Fe(III)-oxide films (see below for moredetailed description).

PASSIVATING IRON OXIDE FILM BARRIER TO FERRATE(VI) PRODUCTION:Application of a filtered, very low ripple, (non variable) DC voltage toan iron anode-based electrochemical cell with strong caustic sodaelectrolyte produces dilute ferrate(VI) coloration of the electrolyte inthe first minutes, and then ceases ferrate(VI) production. In thisevent, a orange-brown, yellow and sometimes black splotchy colored anodesurface is produced. The iron oxide formed in this case does not formferrate(VI) and thickens with time. This iron oxide may be referred toas a passivating layer, and appears to be composed of the Fe(III) oxidesof the type or similar to FeOOH, Fe₂O₃, Fe(OH)₃, and Fe₃O₄).Sufficiently thick iron oxide passivating layers can form in a fewminutes and then always result in no, or just a low concentration of,ferrate(VI) production not useful for even lab-scale preparations. Thispassive layer appears to interfere with the desired ferrate(VI) reactionallowing these other oxides to form, which are unreactive, and hencebecome the final iron product. By increasing the voltage required tooperate the cell only results in undesirable side reactions to occur,such as further buildup of passive oxide layer thickness or oxygen gasgeneration, and then ferrate(VI) production ceases to occur.Accordingly, this need to clean and restart such conventionally poweredcells result in exorbitant production delays, and increases ferrate(VI)production inefficiencies enormously due to the need for many more cellsto be operating, the large amount of labor required for refurbishingpacified cells, and the lower average efficiency of conversion to Fe(VI)product.

Electrochemical Equations Leading to Ferrate(VI) Formation from IronMetal Anode:

While not wishing to be bound by theory, it is believed that thefollowing electrochemical reaction mechanism applies to understandingthe invention of achieving continuous and efficient electrochemicalproduction of ferrate(VI) solutions and compounds. The electrochemicaland other related ferrate(VI) formation chemical reactions are typifiedand described by the following. In a first reaction, associated withholding the electrochemical cell, or just “cell” voltage, Vcell, at ahigh value, Vmax, iron anodes, Fe(0) is converted to several higheroxidation species, including ferrate(VI), Fe(VI), by electrolytic oneand two “electron transfer” reactions depicted as:

Fe(0) → Fe(II) → Fe(III) → Fe(IV) → Fe(V) → Fe(VI) fast fast fast slowfast Reaction No.: R1 R2 R3 R4 R5 No. of electrons 2 1 1 1 1 TOTAL: 6transferred: No. of “4s + 3d” 8 6 4 3 2 electrons for the electronicconfiguration:

Vcell is the measured voltage across a single electrochemical cell ofthe invention, at a particular point in time, and influenced by theapplied voltage, the electrochemical potential of the cell, and anyinternal voltage drops. For example, at high applied voltages, Vcell isVmax at low applied voltages like 0.0, then Vcell varies from Vmax Vmin(curves C and D of FIG. 1A).

Fe(I) is not shown as it is believed not to exist to a significantdegree based on conventional iron chemistry in aqueous, oxidizingenvironments. In addition, Fe(IV) may be produced in a fast reactionfrom Fe(II) [thereby bypassing possibly slow reacting Fe(III) species],by a two-electron transfer, (see below). From conventional coordinationchemistry theory, the molecular geometries of these iron chemicalspecies are expected to be as follows: Fe(0) metallic crystal; Fe(II),Fe(III) and Fe(IV) all six coordinate (octahedral, geometry or Oh);Fe(V) and Fe(VI) four coordinate (tetrahedral geometry Td). Bydefinition, all of these reactions are electrochemical, i.e.electrolytic, oxidation/reduction or “redox”, and disproportionation innature (see below for further illustration and description of thesechemical reactions and how they apply to ferrate(VI) formation). Theoverall driving force for the conversion of iron metal to ferrate(VI)ions is the cell voltage applied externally across the anode andcathode, forming the final product ion, designated Fe(VI), FeO₄ ⁼, orferrate(VI). Based on ligand field theory, such reactions (R1 to R5)would be expected to occur at different rates as the electronicstructures of the ions involved differ substantially from each other,and also since each reaction would vary with cell operating conditions;especially the hydroxide concentration, the total iron concentration,the residence time at the anode surface, the temperature (which affectsboth chemical reaction rates and diffusion rates in the fairly viscousmedium), voltage (Vmin and Vmax), variable DC voltage and currentfrequency, electrolyte flow rate, thickness of anode diffusion boundrylayers internal-cell mixing, and nature of counter ion(s) present.

It is believed the ferrate(VI) production electrochemistry of theinvention proceeds as, or is similar to, that described as follows. Thefast reactions, R1, R2, and R3 proceed rapidly at voltages near Vmax viaone and two electron transfers, to produce an red-orange oxide filmbuildup on the anode surface composed of Fe(III) and Fe(IV), a mixtureof the two, and perhaps mostly Fe(IV), as insoluble oxides. Theconversion of this insoluble film, containing Fe(III) and Fe(IV) oxides,into Fe(V) and Fe(VI) soluble oxoionic species is also believed to occurcontinuously but at a slower rate than red-orange oxide film build-up.This slower rate is believed to be due to the molecular geometry changethat is required to convert from octahedral complexes to tetrahedralcomplexes. To prevent the oxide film from thickening too much, andthereby eventually forming a passivating oxide film of e.g. kineticallyand thermodynamically inert, insoluble, Fe(III) oxides, soluble Fespecies must be allowed to form at a rate similar to Fe(0) dissolutionso that the net effect is to maintain a thin, active iron oxide film.This balance in chemical reaction rates is accomplished in the inventionin two steps; first by adjusting the cell voltage, Vcell, to a valuelower than Vmax which is selected low enough to slow Fe(0) dissolutionto a much slower rate, for example <5%, and preferably <2% of thedissolution rate of Fe(0) at Vmax (as measured by current density andcurrent efficiency with respect to anode weight loss rate), but which isselected still too positive to allow large amounts of Fe(I) to form viareaction R1, as Fe(II) would react quickly with Fe(IV), Fe(V) and Fe(VI)species to form a passivating layer consisting of Fe(III) oxides, ferriccolloids in the electrolyte, and the like. Hence, Vmin is controlledpositive enough to continue conversion of any Fe(II) into Fe(III) andFe(IV) oxide, but not the oxidation of Fe(0) to Fe(II) at a significantrate. Hence, when Vcell=Vmin, the cell current, Icell, is very nearlyzero, i.e. preferable <5% of Icell at Vmax, and preferably <2% of Icellat Vmax, and most preferably <1% of Icell at Vmax.

The beneficial chemical reactions which occur during the Vmin portion ofthe power supply cycle, believed to involve disproportionation redoxreactions, are key to the continuous production of ferrate(VI) as thesereactions allow continuous ferrate(VI) production cell operation bythinning the iron oxide film which thickens during the Vmax phase of thepower signal. This discovery extends the limit of cell operation of theprior art from at most a few hours, to at least several weeks. Althoughthese disproportionation reactions also occur during the Vmax portion ofthe power cycle, they are believed to be in addition to Fe(V) and Fe(VI)direct production at the anode. At Vmin, the disproportionate chemicalreactions convert the insoluble active red-orange iron oxide film intosoluble oxo iron species, and thereby thin the oxide layer. Thisdisproportionation reaction is believed to be one or both of thefollowing chemical reactions. As one disproportionation reaction, it isbelieved that two ions of Fe(IV), present in the active but insolubleoxide film, react with each other by inter-metal ion electron transferto disproportionate into one Fe(V), a soluble oxoanion, and Fe(III), aninsoluble oxide, thereby reducing the amount of iron ions in the film bya large amount, theoretically 50% if the film is mostly Fe(IV) based. Asa second disproportionation reaction, it is believed that two Fe(V)ions, either present from the reactions at Vmax, or from the Fe(IV)disproportionation reaction just mentioned, disproportionate into oneFe(VI) ion, which, being water soluble, diffuses into solution as finalproduct, and into one Fe(IV) species which stays within the solid oxidefilm to react further in the first disproportionation reaction, therebyreducing the extent of film thinning somewhat by the firstdisproportionation (theoretically by 25% on a contained iron ion weightbasis). However, this Fe(IV) so formed, then feeds into the firstdisproportionation reaction mentioned, thereby forming more Fe(V) andFe(III) species, which then forms more Fe(VI) ions, as described, whichdiffuse out of the oxide film into the electrolyte as product, againreducing the oxide film thickness further. This cyclic nature ofdisproportionation redox reactions, occurs due to the presence of twosuch reactions in the same system.

Impact of Above Described Ferrate(VI) Electrochemistry on ViableProduction of Ferrate(VI) Compounds at the Industrial Scale:

Fe(III) oxide films are not highly electrically conducting and are veryinsoluble in water or alkaline solutions used in the electrochemicalgeneration of ferrate. Hence, as is demonstrated in the prior art,conventional electrochemcial conditions result in short-lived cells,which is believed to be due to the accumulation of a non reactiveiron-based oxide film (“passivating layer”) on the anode whichinterferes with electrical current and mass flow to and from the surfaceof the anode. This passive film, which develops quickly, in a fewminutes to 2-3 hours, slows the formation rate, and even stops theformation of ferrate(VI). Therefore prior art methods require thatproduction be stopped after only minutes or just a few, normally aboutthree hours, to remove the highly discolored (black, brown, orange andyellow, with minimal red coloration) film mechanically by wire brushing,using sulfuric acid or hydrochloric acid pickling, or by reverse cellpolarity to discharge the oxide film solid using hydrogen gassing frombeneath the oxide surface (electro-cleaning). Sand blasting was foundineffective in cleaning such surfaces adequately. These refurbishingoperations generate waste solids, waste contaminated pickling acid,waste electrolyte, and/or consume electricity. No practical process forferrate(VI) preparation can be written or produced using the methods ofthe prior art.

The present invention provides a practical and economical methodsuitable for large-scale, continuous, low-cost ferrate(VI) production.This new and useful capability, described in detail below, is a processwith several critical features that can be combined and operated in anumber of ways. The chemistry basis and means of control for thesystematic prevention of the buildup of a passivating film on the anodeallows the continuous production of ferrate(VI) by the cell, and soavoids the wastes of electricity, raw materials, labor, the loss of ironto non-ferrate(VI) byproducts, and the loss of production time, andnonscaleability associated with technology of the prior art. A secondcritical feature is the design of a membraneless cell or undivided cell,which reduces power consumption by at least two thirds, and greatlyextends cell operation life by more than 10 fold by avoiding issuesregarding pluggage of the membrane by Fe(III) oxide solids. A thirdcritical feature is the continuous formation of ferrate(VI) crystallineproduct that can be removed from the electrolyte by continuous,nonmagnetic, solids/liquid separation operations, which thereby alsoallows continuous operation and critical recycle of the highlyconcentrated electrolyte. These critical ferrate(VI) production factorswill be described more fully below. These factors allow the industrialviability of the ferrate(VI) chemical manufacturing process of theinvention in quantities intended to service all scales of needs forferrate(VI), including laboratory quantities, specialty chemicals uses,and large-scale commodity production markets, such as waterpurification, where millions of pounds/year production rates are neededfor each ferrate(VI) manufacturing plant. By operating a process wherethe process components of the invention are incorporated, and more orless continuously operated, provides the continuous or semi-continuousferrate(VI) production process of the invention. This invention permitsefficient large-scale continuous production of ferrate(VI) products, aswell as a means to obtain sufficiently high electrical currentefficiency needed for industrially viable, large-scale ferrate(VI)production

SUMMARY OF KEY PROCESS PARAMETERS FOR FERRATE(VI) PRODUCTION USING THEINVENTION: As per the invention, continuous ferrate(VI) production atscaleable, low-cost conditions is made possible through the use ofcertain process unit operations and process control conditions. Thefirst of these are limitation of the anode surface oxide film thicknessand accumulation rate to that required for high current efficiency andcontinuous operation using a certain variable direct voltage, vDC,resulting in variable direct current, vDI. The second of these keyoperating factors is continuous or semi-continuous harvesting of solidproduct, e.g. crystals ferrate(VI) with counter ion cations of sodium,potassium, lithium, strontium, alkali metal ions, alkaline earth metalion, zinc, calcium, magnesium, aluminum, barium, cesium, and the likeincluding blends thereof. In contrast, prior art methods have usedalternating currents (AC) and direct currents (DC), and AC superimposedonto DC, that do not provide for appropriate reduction of the oxide filmthat readily forms on the iron anode and passivates it, resulting invery short (minutes to a few (<4) hours cell life. Continuous harvestingof ferrate(VI) product enables the electrolyte to be recirculatedthrough the production cell(s) with little or no ferrate(VI) ioncontent, which thereby prevents ferrate(VI) electrochemical reduction atthe cathode, and hence allows the use of a membrane-free cell design.Removing the membrane reduces the power consumption substantially, over60%, reduces the cost of cell fabrication materials by >50%, anddecreases the frequency of cell shutdown for cleaning maintenance fromhours to months, which then almost entirely eliminates total wastesamounts from cell cleaning. Means for achieving continuous harvestingwas not obvious as the ferrate(VI) is produced as a highly water solubleion at the anode, and, should a precipitating agent be added, thenprecipitation would normally occur where the ferrate(VI) ionconcentration is greatest, at the anode. However, precipitation at theanode fouls the anode and eventually fills the anodic compartment of thecell with solids that then requires the cell to be shut down formaintenance. Hence, as part of this invention, the conditions requiredfor ferrate(VI) solid products to form via a controlled crystallizationrate is provided, such that the ferrate(VI) solid product forms from thesurface of the anode and after the electrolyte exits the cell, andbefore the electrolyte is re-circulated back to the cell whilemaintaining low electrolyte volume to anode surface area ratios. Key tothis discovery is that large electrolyte holdup times external to thecell are undesirable as substantial product decomposition then occurs.Hence the product crystallization conditions need to providesufficiently fast crystallization so that the electrolyte can berecirculated back to the cell quickly, to finish oxidation of highlyreactive iron intermediates [believed to be Fe(V) species], yet a highyield of ferrate(VI) recovery is needed to prevent circulation offerrate(VI) past the cathode and causing it to be reduced to magneticby-product crystals, similar, or identical to, magnetite, Fe₃O₄. It wasdetermined that a novel blend of potassium ions, sodium ions andhydroxide ions provide this needed balance of stable productcrystallization without fouling the anode or leaving too high a residualof ferrate(VI). Note that the prior art (minevski) teaches away from theKOH:NaOH use ratio range of this invention as being useful. Thiselectrolyte blend, in combination with the variable DC power signal, thetemperature control profile, and electrolyte internal cell flowcharacteristics are key to high production rates of ferrate(VI) asmeasured by electrical current efficiencies, percent conversion of ironanode into ferrate(VI) product and actual weight of ferrate(VI) per unitarea of anode.VARIABLE DC (VDC) POWER SIGNAL DESCRIPTION FOR THE INVENTION, ANDDETERMINATION OF ACCEPTABLE Vmax AND Vmin SETTINGS FOR A PARTICULAR CELLDESIGN: As used herein, DC, stands for direct current and has themeaning usually associated therewith in the art. For vDC, the voltagewill typically swing between minimum (Vmin) and maximum (Vmax) valueswith the absolute values dependent on cell design and operatingsettings, especially the temperature, anode-to-cathode gap, theconcentration of caustic, the caustic cation used (normally potassiumion, sodium ion, lithium ion, and the like and/or blends thereof, seeabove) precise morphological structure of the iron anode, the cathodematerial, and the cathodic reaction normally exhibits hydrogen gasformation from water, and so on.

[2H₂O+2e ⁻→H₂(g)+2OH⁻]  (1)

However, for the invention, once set for a particular cell design, powersignal frequency, and operating conditions, the vDC voltage does notswing substantially below Vmin or above Vmax.

Vmin is controlled just low enough to slow or stop the dissolution ofiron metal anode, to suppress the formation of Fe(III)/Fe(IV) oxidelayer thickness, but high enough to maintain oxidizing conditions at theanode surface to prevent side reactions taking place, especially thereactions between the product, ferrate(VI) ions with reduced forms ofiron, i.e. metallic iron and/or divalent Fe, Fe(II) which would quicklyresult in formation of a passivity layer of Fe(III) oxide on the surfaceof the anode. Therefore, Vmin is controlled low enough to prevent theoxidation of more Fe(0) from the anodes surface, and this is indicatedby the overall cell current, Icell, dropping to, or near to, zero, orabout 1% of the value of Icell at Vamx, or at most about 5% of thisvalue. Vmin set in this manner prevents significant additionalthickening of the oxide film during the Vmin cycle of the power signal.Therefore, Vmin is the voltage across the anode and cathode above whichthe conversion of Fe(0) to Fe (III) and Fe(VI) oxides isthermodynamically favored, but is very slow kinetically, so that oxidefilm formation is substantially depressed. As current density, thissetting of Vmin corresponds to about 0.01 to 1.0 mA/cm². In chemicalreaction form, Vmin is the cell voltage needed to substantially preventthe following spontaneous redox reactions from occurring:

Fe(VI)+Fe(0)→2Fe(III)  (2)

-   -   passivating film    -   and/or

Fe(VI)+3Fe(II)→4Fe(III)  (3)

These reactions are prevented by passivating film having just barelysufficient voltage present such that if any reactions involving Fe(0) orFe(II) occur, that they are oxidizing reactions, so that only Fe(III) orhigher oxidation states of Fe can be formed from Fe(0) and Fe(I).However, as the voltage is just barely sufficient to drive thesereactions, very little reaction occurs in the frequency cycle of Vmin asis measured by the current density at this time (see above). During thistime chemical reactions other than electrolytic reactions can occur (seebelow), allowing the oxide film to thin as desirable ferrate product isformed.

At the Vmin conditions described above, important spontaneous(thermodynamically allowed) intra-film electrochemical conversionscontinue, especially disproportionation reactions. It is believed thatthe invention uses such chemical reactions to maintain a thin,non-passivating, red-orange, active, iron oxide film on the surface ofthe anode.

Vmax: Vmax, as used herein, is the voltage across each anode and cathodeof the invention that is at or above the voltage and current densitywhere the iron anode dissolves electrolytically at a fast rate, andwhere the lower oxidation states of iron, Fe(O) through Pe(V), areconverted to Fe(VI) quickly. Vmax is determined and set for the cells ofthe invention as that voltage determined for the particular cell designand set of operating conditions, manifested as a “flattening out” of avDC or AC electrical power supply signal wave, e.g., a sine wave,saw-toothed wave, or other voltage vs time wave pattern, placed acrossthe cell at otherwise operating conditions. In actual practice ofmaximizing ferrate(VI) production by the invention, a square wave powersupply signal is preferred since it maximizes the time spent at Vmax,where most of the ferrate(VI) production is occurring and minimizes itelsewhere. However, a simple rectified AC single at the frequency of theutiLity supply, without filtration, is a preferred source of vDC due toits ready availability, low cost, and simplicity of use.

An intermediate simplicity and cost power signal of vDC to generate Vmaxand Vmin settings to practice this invention can also be prepared bysuperimposing a high current DC (offset) voltage (from a DC power sourceof any kind) onto a high current AC wave provided, most preferably in aratio where the resultant vDC voltage never drops to below zero in thevoltage vs time plot. As is well known in the art, such voltage withfrequency profile plots are readily measured, characterized, adjustedand monitored using an oscilloscope. The oscilloscope trace then is alsouseful in screening candidate power supplies and power voltage wavesignals for those that meet the criteria of this invention, fordetermining, setting and measuring Vmax and Vmin values, and fordetermining the optimal vDC frequency.

ELECTROCHEMISTRY OF FERRATE(VI) PRODUCTION IN RELATION TO VARIABLE DCPOWER SIGNAL: Referring to oscilloscope tracings in FIGS. 1A and 1B, thepresent invention uses a power supply to an electrolytic cell consistingof a variable voltage wave of any type (G) of direct current (DC),symbolized as vDC in this application, that varies at a certaincontrolled regular or irregular frequency between a maximum voltage,Vmax (B), at which the iron anode dissolves and ferrate(VI) is producedalong with intermediate oxidation states of iron, and a minimum voltage,Vmin (F), set at zero or, preferably, above zero but at a voltage whereVmin is ≦Vmax, and preferably at a value of Vmin in which iron anodedissolution is essentially stopped, or slowed substantially, relative toits dissolution rate at Vmax, and where the oxide film, formed on theanode during the period the cell voltage (Vcell (G)) is at Vmax, isallowed to thin down by disproportionation. Ferrate(VI) production isobserved to occur mostly during the time periods when Vcell=Vmax (B) bydirect electrolytic oxidation of Fe(0) to Fe(VI), where it is believedthe Fe oxidations states of Fe(II), Fe(III), Fe(IV) and Fe(V) areinvolved (Reactions R1 through R5). An insoluble solid oxide film alsoforms during this time that is believed to be mostly comprised ofinsoluble oxides of Fe(III) and Fe(IV) (see below). If Vcell (G) ismaintained constant, for example at about Vmax (B), then this oxide filmwill thicken to the point of causing anode passivation whereatferrate(VI) production is substantially reduced and can cease formationaltogether. Such low levels of ferrate(VI) production are nonviable forindustrial-scale production of even specialty quantities of ferrate(VI)salts, much less commodity quantities. However, in the currentinvention, it was found that, by frequently adjusting Vcell (G) to Vmin(F), e.g. by using vDC at about 0.001 to 240 Hz, and preferably 0.01-120Hz, and most preferably 0.1-60 Hz, then disproportionation (selfelectron exchange) chemical reaction (s) occur alone, and/or incombination with other electrochemcial reactions, thereby forming watersoluble oxoanions of Fe(VI) and Fe(V), (i.e. FeO₄ ²⁻ and FeO₄ ³⁻respectively), which diffuse away causing the iron oxide film on theanode to thin down preventing it from reaching passivating thickness,which would cause stoppage of ferrate(VI) production. It is believedthat the electrochemical reactions which occur during the Vmin period(t₂ and t₃ of FIG. 1A) thins the oxide film on the anode by formingthese water soluble Fe(V) and Fe(VI) ferrates which then can diffuseaway from the anode and into the electrolyte, where the ferrate(VI) canbe induced to crystallize into a solid that can be harvested. WhileVcell=Vmin, dissolving significantly more iron metal from the anode isprevented, which thereby avoids formation of more iron oxide film, whichwould give rise to eventual passivation thickness. Therefore thesecondary ferrate production chemistry (disproportionation) results inmaintaining a thin and conductive “active” oxide film, and henceoffering the continuous production of ferrate(VI) capability,representing the breakthrough that is a significant part of theinvention (along with continuous product harvesting process technique).

While not wishing to be held to the specific colors of oxide surfacefilms on steels as being significant, experimentally it was invariablyobserved that the desirable non-flaking film appears as a uniformred-orange color, and not black, brown, yellow or rust orange in color,nor are there heavy deposits of precipitate solids present on thisdesirable red-orange film. Therefore, it appears that this red-orangecoloration of the anode indicates a viable surface for ferrate (VI)production.

Therefore, it is believed that the chemical reactions which occur duringthe Vmin portion of the power cycle (t₂ and t₃) do not requireelectrolytic reactions at the anode as the observed cell current duringthis part of the cycle is minor, i.e. <5%, and normally, <2%, and often<1%, of the total current flow at Vmax. Therefore, it is presentlyconcluded that the anode oxide film reactions during the period oftransition from Vmax to Vmin (curves C and D of FIG. 1A) involves ironmolecular species produced during the electrolytic oxidation of theanode while the cell was at Vmax (FIG. 1A, curve B).

As FIG. 1A, regions (A) and (B) indicate, if one attempts to provide aDC voltage higher than Vmax, i.e., that at which ferrate(VI) productionoccurs, the observed and measured voltage will be appear to be “cutoff”, i.e., held at an essentially constant or a “buffered” value (B).Ferrate(VI) product has been found to be produced at a rate directlyproportional to the total flow of electrical DC current being deliveredby the power supply. As an example, region (A) of FIG. 1A above voltagelevel Q is believed to drive increased iron anode dissolution to formboth active iron oxide film and soluble ferrate(VI). This increasingreaction rate with increased total cell current, Icell, at a constantvoltage effectively increases the current density (and hence theapparent conductivity at the anode surface), thereby, stabilizing thevoltage which appears as a flat-topped wave (B) in the oscilloscopetrace.

Specifically, referring to FIG. 1A, in a preferred case, the appliedvoltage, Vcell (G), is increased in roughly square wave form (A), andbecomes constant as the voltage increases to Vcell=Vmax, (B). Duringthis voltage sweep ferrate(VI) production commences at voltage level Q,and is believed to be fastest when Vmax is reached. The Vmax voltage ismaintained for a selected time (t₁), set by the wave form frequency, andthen dropped to Vmin (Curve C and D of FIG. 1A) over periods t₂ and t₃,which essentially results in a zero electrolytic current, i.e., <5% andnormally <1% of the total current flow at Vmax. Note however, that ithas been discovered that the voltage does not drop off in the form ofthe wave that is applied (Curve C, C′ and A′). This is true regardlessof the shape of the applied signal, which can be a square wave, sinewave, rectified AC, or saw-toothed wave, combinations thereof, and thelike. Rather the drop off is delayed as shown by the Curve D after timet₂, essentially an exponential decay in Vcell with time t₃ (Curve D,starting with point Q and ending at point F). This observation isinterpreted as indicating that electrochemical (not electrolytic)reactions continue when the applied DC voltage is reduced by the powersupply (Curve C) at the selected frequency and waveform as vDC. Thus,instead of following the applied voltage (Curve C plus C′), instead themeasured voltage drops exponentially to one level (Curve C) duringperiod t₂, then asymptotically to the still lower voltage (Curve D)during period t₃ to Vmin point (F) and then remains essentially constantat A′ (observed but not shown in FIG. 1A). The voltage then increasesagain as shown by curve A′ during the next voltage pulse giving thevoltage sweep A then B. Note that no, or at most a small amount, offerrate(VI) is made if nonvariable (filtered) DC is employed at Vmaxvalue B for t₁. It is important that the voltage be allowed to falllower than point Q in order to achieve thinning of the iron oxide filmto prevent passivation, which also produces more ferrate(VI) (see belowfor proposed mechanism). Note that at certain cell operating conditions,e.g. FIG. 1B, Curves C and D can blend together somewhat. FIG. 1Billustrates a square wave type used fir an electrochemical cell wherethe change at the Q point is not so apparent (see FIG. 1A). However, anapproximate decrease in voltage to a value of 1.08V from Vmax of 2.3over about 500 is still very pronounced, and found to be markedlydifferent than the cell-free reference case where a 1 ohm resistor isused in place of the cell. Vmin is still determined and set in the samemanner as for FIG. 1A. The appearance of critical Curve D can normallybe adequately deconvoluted from Curve C′ by collecting a referenceoscilloscope trace in which a resistor, e.g. of approximately one ohm orless, and rated to carry the expected current of the power supply, andthen comparing this trace to the Curves obtained with the cell in linein place of the resistor.

To determine Vmin, using the power supply of the invention, FIG. 2,Vcell (G) can be reduced from Vmax such that current flow is less thanabout 10% of the current at Vmax, preferably <5%, and most preferably<about 1%, of the cell current (Icell) measured at Vmax. Typically, themaximum length of time that the voltage is at Vmax t₁ is about 1 minuteand the minimum time is about 0.001 seconds. Generally, the frequency(peak to peak) of the direct current pulses, A, is between about 0.001Hz and 1,000 Hz. A typical wave has a frequency of about 0.1 Hz to about480 Hz, more typically about 0.1 Hz to about 240 Hz, and even moretypically about 0.1 to about 120 Hz. The allowed time at Vmin (F) isheld as short as possible, typically about 0.01 to 0.2 seconds.Frequency of the power signal is set such that the exponential drop inVcell from Vmax (Curve C plus D, t₂ plus t₃) is just completed or isnearly complete, i.e. complete being the point where the voltage readingno longer changes over a period of tenths of seconds to seconds (e.g.set to where at least 80 and preferably more than 90% of the change toVmin has occurred, or most preferably precisely at the time where theminimum Vcell voltage is equal to Vmin but not longer Point F). Theperiod of time at Vmax (t₁), t₄, and less than Vmax (t₂ and t₃) do notneed to be equal, and in fact should be optimized separately such tomaximize ferrate(VI) production rate. In selecting the period forVcell<Vmax, t₂ is kept as short as possible, about <80 msec, and thetime allowed for equilibration D, t₃ is adjusted just sufficiently longto provide the maximum ferrate (VI) production efficiency overall andespecially at Vmax (since time t₃ controls buildup thickness of theactive iron oxide layer on the anode during the Vmax portion of thepower curve). This control of active iron oxide layer buildup means thatthe thickness of the oxide layer is reduced during the time t₃ whereasit thickens during time t₁.

During the Vcell drop-off period between Vmax and Vmin (t₂ and t₃), itis believed that iron anode dissolution rate is reduced or ceases;however, the spontaneous disproportionation chemical reaction(s)involving Fe(V), and perhaps Fe(IV), to form ferrate(VI) product, FeO₄⁼, and perhaps other such reactions, continues. This continued reaction,that produces soluble ferrate(VI), occurring without the additionaldissolution of iron anode material at the high level of applied voltage(Vmax), causes a reduction in the thickness of the active Fe(III) andFe(IV) oxide film on the anode preventing formation of a passivatingfilm. After the film thickness has been reduced a selected amount,[which is set by the equilibration time t₃ selected to match the cellconditions in use, and is measured as the exponential drop in theobserved cell voltage (Vcell) during period t₃, to a constant, or nearlyconstant cell voltage (Vmin, Point F), and whose duration is controlledby DC offset and vDC frequency settings], the voltage A is reapplied asper the set frequency of the power signal where it is ramped up to VmaxB, allowing the cycle of film growth and thinning to be repeated at thepreset frequency. In this manner, continuous ferrate(VI) production isachieved and buildup of passivating iron(III) oxide film is avoidedresulting in little net change in oxide film thickness over minutes,hours, days and weeks.

Role and Control of Reactive Intermediates in the Production ofFerrate(VI) Using the Invention:

Although not wishing to be limited in this invention by theory, it isuseful to have a theoretical basis of understanding of chemicalprocesses to understand the importance of certain parameters and processbehaviors. In this vein, a theoretical basis is provided here forimportant roles for certain highly reactive chemical intermediates thataffect ferrate(VI) production using the method of this invention. Suchspecies, most likely consisting of insoluble Fe(IV) hydrated oxide andsoluble Fe(V) oxoanionic species, are believed to be involved as highlyreactive intermediates in the production of Fe(VI) product, though theyare always present at very low levels at any particular time. Suchreactive intermediate chemistry behavior is well known in the science ofchemistry, being a factor in most chemical reactions. Fe(V) and Fe(IV)axe believed to be involved in the production of Fe(VI) product usingthe invention in a manner similar to the following proposed mechanism:

PROPOSED ELECTROCHEMICAL OXIDATION AND DISSOLUTION CHEMICAL MECHANISMFOR THE PRODUCTION OF FERRATE(VI) ION FROM Fe METAL: While not wishingto be bound by theory, the following model is proposed for theelectrochemical production of ferrate(VI) using the electrochemical celldescribed above and in the Examples in which cells of anode areasranging from 5 cm² to 866 cm² are described. As is well known in theart, the total cell current, Icell, needs to be increased or decreasedlinearly with anode surface area, as the total cell size is changed upor down, respectively, to retain the desired current densities at theanodes. However, Vcell values are kept essentially constant withscale-up to maintain the same electrochemistry. This is true whether theanodes are single or of “stacked” cell design or whether they are dipoleor monopole design. Two sets of chemical reactions are important and areshown below, those that occur during the Vmax portion of the powercycle, t₁, and those that occur during the Vcell<Vmax portion especiallyto t₃ and point F.

To help understand why variable DC voltage according to the presentinvention is effective, a chemical reaction sequence is set forth below.Two sets of reactions are important; the first set, electrochemicaloxidation and dissolution, corresponds to the period when the voltageand current density are high (t₁, Vmax, Imax), and the second set,disproportionation and dissolution, corresponds to when the voltage andcurrent density are at the minimum values (Vcell <Vmax, t₂, t₃, point F,Vmin, Imin).

Reaction Set 1: Electrochemcial and Other Chemical Reactions that occurduring the Vmax(t₁) Portion of Cell Power Cycle(Conditions: Power signal: vDC conditions, undivided cell, Vmaxapproximately 2.14 volts, preferably 2.3 volts, anode area: 3-1000 cm²,Icell 2-100 A, preferably 50-55 Amps, and anode area=866 cm² currentdensity at the anode: in the range of 1-100 mA/cm² preferably 35 mA/cm²(where anode surface area or Icell are varied such to achieve thesecurrent densities), Tcell: 10-70° C., batch or continuous flow-throughelectrolyte. Electrolyte: 17-52% NaOH with or without added KOH or otherhydroxide salts):

Chemical “Half-Cell” Reaction Estimated Relative (waters omitted forclarity)^(d) Reaction Rate Rx. # Fe⁰ + 2OH⁻ → Fe^(II)(OH)₂ ^(o) + 2e⁻fast to very fast (4)^(a) Fe^(II)(OH)₂ ^(o) + 2 OH⁻ → Fe^(II)(OH)₄ ²⁻very fast (5)  Fe^(II)(OH)₄ ²⁻ → Fe^(III)(OH)₃ + e⁻+ OH⁻ fast (6)^(a)Fe^(III)(OH)₃ + OH⁻ → Fe^(IV)(O)(OH)₂ + slow to rapid^(c) (7)^(a) e⁻ +H₂O Fe^(II)(OH)₄ ²⁻ → Fe^(IV)(O)(OH)₂ + fast  (8a)^(a) 2e⁻+ H₂O {and/orFe^(II)(OH)₂ + H₂O→ fast  (8b)}^(a) Fe^(IV)(O)(OH)₂ + 2e⁻Fe^(IV)(O)(OH)₂ + 4OH⁻→ Fe^(V)O₄ ³⁻+ slow (RDS)^(b) (9)^(a) e⁻ + 3H₂OFe^(V)O₄ ⁻ → Fe^(VI)O₄ ²⁻ + e⁻ fast #(10)^(a) ^(a)Rate determined byapplied current density ^(b)RDS = rate determining step due to geometrychange ^(c)This may be a slow reaction depending on degree of hydration,oxide film thickness, etc., of the anode surface iron oxide film (seetext) ^(d)Underscore indicates solid oxide film phase component onsurface of anode

Followed by,

Reaction Set 2: Disproportionation and Dissolution of Active Red-OrangeIron Oxide Layer on the Anode That Occurs During Vmax and Vcell<Vmax toVmin (t₁, t₂ to t₃) Portions of the Cell power Cycle.

[Conditions: Voltage of the vDC signal ranging from Vcell<Vmax to theVmin condition: e.g., Vcell at about 0-2.1 volts, preferably 1.7 volts,with Icell=0.001-1.0 A, (i.e. very low current density but not zero).Other conditions are the same as Reaction Set 1.]

Chemical Reaction Estimated Relative (Showing only oxidation statechanges) Reaction Rate Rx. # 2 Fe(IV) = Fe(HI) + Fe(V) slow to medium(11) 2 Fe(V) = Fe(IV) + Fe(VI) slow to medium (12)The chemical species are of the same representative formulas given inreactions (6) through (10).

In the above reactions, Sets 1 and 2: a) water molecules omitted forclarity, b) RDS is the slowest or “rate determining” step due to thechange in molecular geometry from six coordinate (Oh) to four coordinate(Td), c) Conditions disfavoring reaction (6) and promoting reaction (8aand 8b) are believed to be desirable by avoiding the formation ofpossible slow reacting Fe(III) species which might slow the dissolutionrate of the active film.

The above-proposed chemical mechanism is consistent with the operatingcell data and observations of the invention for both batch andcontinuous cell operational modes. Reaction (8) is preferred over thecombination of reactions (6) and (7) if reaction (7) is slow (which ishighly probable if slowly reacting iron(III) oxides, vs. hydroxide,species are formed). That reaction (10) may be the main path for ferrateproduction is supported by invariant observations that exceeding athreshold minimum vDC voltage/current density appears to produceferrate(VI) at a fast rate, and below this vDC voltage/current density,the rate of ferrate production does not appear to proceed rapidly, if atall. These conclusions apply to both batch and continuous cellconfigurations and, were used to establish the continuous productionconditions described herein that have proven to be very robust,including demonstrations that the cell of the invention is not prone topassivation at these conditions, and readily recovers should process“upsets” occur, i.e. power failure, or electrolyte flow stoppage.

Reactions (11) and (12), correspond to thinning of the active iron oxidesurface film, created while the cell is at the Vmax condition [reactions(4), (6), and (7)], because the Fe(V) and Fe(VI) products arewater-soluble and diffuse away from the film. Therefore, this activefilm is proposed to consist of a combination of Fe(II), Fe(III), andFe(IV) oxyhydroxides which form in reactions (4), (6), (7) and (8).Therefore, by allowing reactions (9) through (12) to occur, where onlyreactions (11) and (12) occur at Vmin<Vcell<Q, the forming of a thick,passivating oxide film, by reactions (7) and (8), is avoided.Significantly, at the cell condition of the invention, if such apassivating film is formed, then it is observed that the electrochemicalpower drives formation of a thicker oxide film rather than oxygen gasproduction, and the film loses its uniform red-orange color, developinginstead blotched colors of rust, brown, black, and orange and yellow.This observation is interpreted as being due to the passivating filmbeing poorly electrically conductive but of sufficient water porositysuch that the iron metal dissolves to form more iron FeIII-based oxidebeneath the passivating film Reactions 4, 5, and 6), thereby thickeningthis barrier film. That very little, if any oxygen is formed at theanode at the described cell design and operating conditions, is viewedas a significant beneficial characteristic of the cell of the inventionas such gas production would represent a competitive consumption ofelectrical power to that used for ferrate(VI) product production.Without the tendency for oxygen gas production theoretical electricalcurrent efficiencies can approach 100%. It also removes opportunities toform explosive gas mixtures. Current efficiencies of up to 60% have beenobserved in the operations of the cell of the invention (based on theamount of ferrate(VI) formed per electron applied to the cell). As iswell known in the art, optimization of the cell designs and operationalparameters described in the invention would be expected to increase thecurrent efficiency values still further, and could be expected to reach90%, and perhaps >95%. A current efficiency of only about 1.0-20% isneeded for the invention to provide commercially viable specialtychemical ferrate(VI), and at least 20-30% to provide a large volumecommodity chemical product. With the invention laboratory samplepreparations are practical at current efficiencies of 0.1-10%.

ROLE OF HYDROXIDE ION: The role of hydroxide ion or caustic is clearlyseen in the above-described mechanism because the formation of oxidesrequires OHf ions, and the prevention of reactive protonated species(e.g., HFeO₄ ⁻) avoid product decomposition, thereby stabilize theproduct and favors the formation of soluble oxo-ions over hydroxidecompounds. The caustic increases the solubility, and hence the mobilityand reactivity, of FeIII and FeIV ions, normally highly insoluble,through anionic complex formation reactions, i.e.,

Fe^(III)(OH)₃+OH⁻→Fe^(III)O(OH)₂ ⁻(pH>13)  (13)

-   -   insoluble film soluble ion    -   and

Fe^(IV)(O)(OH)₂+OH⁻→Fe^(IV)O(OH)₃ ⁻  (14)

-   -   insoluble film soluble ion        These formulas are illustrative only. Specific molecular        formulas can vary by water content, deionization, and in other        ways.        Depending on the temperature/time/concentration profiles, the        Fe(III) species may be best represented as Fe(OH)₃ or as FeOOH.        These are taken as chemically equivalent in the above mechanism        discussion. Note also that Fe^(III) oxidation state notation        type is equivalent to Fe(III) notation type for all Fe species,        as persons skilled in the art will know.

IMPORTANCE AND RATIONALIZATION OF SYSTEM ELECTROLYTE VOLUME TO ANODESURFACE AREA KEY PARAMETER FOR ENHANCING ELECTRICAL CURRENT EFFICIENCY:While not wishing to be bound by theory, it is believed that thepresence of the soluble Fe(V) species explains why the anolyte volume toanode surface area ratio (Velecnolyte/Aanode, or just “V/A”) is arelevant process control parameter, and certain aspects of productreactivity. This V/A ratio determines the time that the reactive speciesis allowed to exist outside the cell until it is re-introduced to thecell so that it can be further oxidized to Fe(VI) by Reaction (10).Based on this, a second cell can optionally be included in the anolyteexit electrolyte stream to complete the oxidation of to Fe(V) to Fe(VI)product [Reaction (10)] using a non-sacrificial anode such as Pt on Ti,carbon, a dimensionally stabilized anode (DSA), and the like. Additionof a small amount of strong chemical oxidant (e.g. persulfate,monopersulfate, oxone, hypochlorite, chlorine, and the like) is alsoeffective in completing reaction 10 in the electrolyte emerging from thecell. The polishing cell is most preferred and chemical oxidant additionleast preferred.

Therefore, on exiting the production cell, the electrolyte carryingsoluble Fe(VI) and a small amount of Fe(V) may optionally be sentthrough a “finishing” or “polishing” cell to convert at least a part ofthis Fe(V) into Fe(VI) product. If this is not done, the water-solubleand reactive Fe(V) (“reactive intermediate”) may react with hydroxideion or water to produce colloidal Fe(III) particles, e.g.

Fe(V)+2OH⁻→Fe(III)+½O₂+H₂O.  (15)

or some other reaction. This Fe(III) oxide colloid can be filtered outof the electrolyte using a filter of about 10-micron porosity. However,it is preferred to avoid such Fe(III)-based by-product as any oxidation,i.e. Fe(V), loss represents a decrease in current efficiency. As analternative to, or in addition to, the use of a finishing cell, smallamounts of a strong chemical oxidant can be added, such as hypochloriteion or monopersulfate ion, etc. For example 50 cc of aqueous 5% NaOCl issufficient to treat 700 ml of anolyte. Note that, if chloride ion orhypochlorite ions are undesirable to have in the crystalline ferrateproduct, then the ferrate product can be cleaned of such impurities byone or more recrystallizations.

Another side reaction at the anode surface that is minimized by theinvention is

Fe(VI)+Fe(0)→2Fe(III) (e.g. as Fe₂O₃)  (16)

because this Fe(III) product rapidly forms a film that passivates theanode. Hence, it is undesirable to stop the variable DC power to thecell at any time after ferrate(VI) production is in progress. SuchFe(III) oxides are not reactive (as noted above) but can be removed orconverted to ferrate(VI) producing the red-orange active film using thevariable DC power supply conditions disclosed by this invention. Hencerecovering ferrate(VI) production without having to dismantle and cleanthe cell and electrodes of passivating film is a significant benefit ofthe invention.

EFFECT OF TEMPERATURE: For the invention, temperature control optionallycan be used to increase electrical current efficiency for ferrate(VI)production by using it to limit Fe(VI), and perhaps Fe(V), losses byside reactions, speed the ferrate production reaction rate, and increasediffusion rates. The latter reduces both total cell voltage (and henceit decreases power consumption), and increases diffusion of Fe(VI)product ions from the anode surface. By increasing Fe(VI) productdiffusion rates, the Fe(VI) concentration, [FeO₄ ⁼], next to the anode,the location where [FeO₄ ⁼] is always the greatest (its point ofproduction), is kept at a minimum. Keeping [FeO₄ ⁼] at a minimum isbeneficial as it increases electrical current efficiency by minimizingFe(VI) losses due to decomposition by self reaction, which is known tobe second order in [FeO₄ ⁼], i.e.

2FeO₄ ⁼+3H₂O→2FeOOH+ 3/2O₂(g)+4OH⁻  (17)

Since reaction rates, such as this reaction, increase with increasingtemperature, there will be an optimum temperature affect. Thus, anincrease in temperature avoids side reactions with the anode electrodeby providing increasing electrical current and efficiency, ferrate(VI)ion diffusion rates from the anode surface, and avoids deposition ofbyproducts, such as FeOOH. Effective temperatures for cell operation areabout 10° C. to about 80° C. More preferred are temperatures of about25° C. to about 50° C., while most preferred are temperatures of about40° C. to about 45° C. On the other hand, decreasing temperature of theanolyte upon exiting the cell is believed to be beneficial forincreasing cell electrical current efficiency by decreasing the rates ofall Fe(V) and Fe(VI) decomposition reaction pathways, and also thesolubility of the recovered product salt, e.g. Na₂FeO₄, K₂FeO₄, SrFeO₄,BaFeO₄, ZnFeO₄, MgFeO₄, CaFeO₄, and/or Li₂FeO₄, and/or mixtures, blends,double salts, and hydrates thereof, and the like.

TOTAL CELL VOLTAGE (Vmax): For the membrane-less cell of the invention,the range of acceptable Vmax voltages are 1.7-4.0V, preferred is 2.0 to2.5V, and most preferred is 2.5-2.9V. It is believed that the lowestvalue for Vmax is largely limited by the reaction chemistry for theentire cell (both anodic and cathodic reactions), and the voltagepredicted by the Nernst equation, corrected for internal cellresistances. Although desirably low, this voltage range was found to besufficient to drive the ferrate(VI) production desired reaction, to givehigh current efficiencies, and provide a low hazard process.

FIG. 2 illustrates on embodiment of a typical apparatus, 200, forsupplying a variable D.C. voltage to an electrolyte cell according tothe invention. Frequency generation, 210, provides a selected waveshape,e.g. sine wave, square wave, saw-tooth wave, or a custom generatedwaveshape to control circuit, 220, via signal, line 215. Controlcircuit, 220, adds offset voltages (DC) and provides the necessarysignals via signal line, 225, to high amperage DC power supply, 230.Typical control signals provided by control circuit, 220 includedwaveshape, vDC offset voltage, frequency control, and voltage levels.The DC power supply generates the selected DC potential that is placedacross the ferrate production cell 240 via line, 235. DC power supply230 is selected to provide the current and voltage required by the cell,240, typically 1-500 A (more precisely 1-200 mA/cm² of anode), and 0-5volt cell. Bipolar cell arrangements will require higher voltage, thatis approximately Vmax times the number of individual cells.

A lower Vmax was determined to be effective (Vmax=1.7V) by dropping thetotal current to ⅓ of the power supply maximum for the cell (about 15 A)with an anode surface area of 866 cm² (17.3 mA/cm²) described below.This change resulted in elevated current efficiency for the 8 hours ofthe test. One tenth of this current (about 1.7 mA/cm²), was also foundto be effective in generating ferrate(VI) according to the cell of theinvention for a period of more than two weeks in which very little ifany by-products were observed. Low voltages are desirable from aneconomic standpoint because power costs are proportional to voltage[P(watts)=I·V]. It is well known in the art that using a lower voltagewill result in a significant reduction in energy costs. This is a majorbenefit of the invention as the many industrial applications forferrate(VI) require commodity chemical pricing. The production rateincreases in proportion to overall cell current, I cell, and does so ina linear (straight line) fashion if current efficiency is constant, oris curved if current efficiency changes as the current density increasedor decreased. Hence, an optimal current density is to be used to balancethese two effects in order to provide the greatest current efficiencyand production rate for ferrate(VI) manufacture in those cased requiringthe lowest product manufacturing cost. For the cell of the invention,the functional current density range is 1-200 mA/cm², preferably 2-80mA/cm², and most preferably 20-60 mA/cm².

Using the 866 cm² anode example (see below), the DC current sweep forthe vDC was typically from 1 to 53 A (the maximum output current allowedby the power supply). During one test, the maximum current was decreasedto 17 A. This decrease in current resulted in a proportionally lowerproduction rate of ferrate(VI). However, since the amount of currentpassed to the cell was lower, the resulting current efficiency remainedthe same.

Variations in the frequency were also tested using a power circuit ofthe type shown in FIG. 3. FIG. 3 illustrates one typical control circuit220 that is useful with the invention. However, it is noted that thecontrol circuit may be any off shelf unit that provides the controlsignals selected for the power supply 230. Power supply 310 typicallyconnected to 115 AC has three outlets, one for +24 DC, a second forground (OVDC), and a third for −24 vDC. Voltage control unit 320 (type7815) provides a positive voltage for pins #7 for operational amplifiers350, 352, 354 and 356 (all type LM741). Voltage control unit 2022 (type7915) provides a negative voltage for pins #4 on operational amplifiers350, 352, 354, 356. Voltage control unit 324, 326 (type 7805 and 7905)provide a positive/negative voltage reference (respectively) to thevoltage offset control 330. Offset control 330 provides the voltageoffset for controlling the minimum voltage applied to the ferrateproduction cell. Input from the frequency generator for frequency,waveshape, and the like is received at generator input 338. Amplitude ofthis input signal is controlled by amplitude control 340. The controlsignal for the power supply 230 is provided at output 360. Variousresistors (R1—R8) and capacitors (C1-C2) were used with their valuesindicated in FIG. 3. Control circuit 220 was that actually used fortests herein.

Another embodiment for a control unit is shown in FIG. 4. Although notused yet this controller can be used to provide the voltage control toelectrochemical cell 240. A twelve volt power supply 410 connected toexternal 115V AC provides ground and a 12V supply voltage. Voltagecontroller 420 (type 7805) provides output voltage control tomicroprocessor 16f.876. Voltage controller 422 (type 7805) provides avoltage signal for variable resistors 424 and 426. Voltage amplitude isadjusted at variable resistor 424 and voltage offset at variableresistor 426. Operational amplifier 430 provides the amplitude signal tothe microprocessor. Operational amplifier 432 provides the offset signalto microprocessor 460. Operational amplifier 480, 482 (type LM6032) andprocessor 484, 486 (type 4012) provide waveform and frequency control.The units set and reset via switches 490 and 492 respectively. Atransistor 494 (type 2203) provides power control. Signals to theexternal power supply 1330 are provided at output 496. Theelectrochemical cell is connected across points A and G.

In one series of tests, the frequencies used were 0.02, 0.5, 1, 2, 2.5,and 5 Hz. Under the conditions tested, no clear trends were observedwith these variations and, hence the full range of frequencies waseffective. Other tests used 60 Hz and 120 Hz, which were also foundeffective. Waveform appeared to have some impact on current efficiencybut still minor. For the equipment used and the range of frequenciesperformed in the tests, it appears that for a given waveform withsimilar voltage characteristics, current efficiency increases withfrequency. However, note that optimal current efficiency values dependon specific values of Vmax, waveform and current density used.

With the teachings of the present invention, a person skilled in the artwill be able to perform routine statistically designed ferrateproduction rate and current efficiency optimization tests to identifythe most efficient combination of power supply frequency, currentdensity, and waveform, Vmax, and Vmin profile. Importantly, very goodcurrent efficiencies and selectivity have been demonstrated.

The observation is that ferrate(VI) formation only occurs during anarrow voltage range. The experiments indicate that it is most preferredthat Vmin should never be zero or below (i.e., power forms not desiredare alternating currents (AC), or AC superimposed on DC such thatVac>Vdc, to avoid product decomposition and passive film formation). Thedata for Vmin of about 1.7 volts suggest that reactions (4), (5), and(6), and probably (7) and (8), are always kept maintained reacting leftto right by the preferred cell operating Vmin voltages, and are nevercompletely stopped, or allowed to reverse by lowering the voltage belowVmin, and especially by lowering Vmin to less than 300 volts. Reactions4 through 12 therefore correspond to the low level of electrolyticelectrochemical change occurring at 1 min.

One means to refine the settings for Vmax and Vmin, regardless ofwaveform, and using the vDC high amperage power supply described, theprocedure is to power up the cell within the above voltage, currentdensity, and temperature ranges for any cell designs of the invention,is to then use the control circuit (FIG. 2) to adjust Vmax, DC offsetvoltage, and frequency to maximize the curve portions D and B (FIG. 1A).

Electrolyte Composition:

The electrolyte is typically an alkaline solution of a metal ionhydroxide or metal ion hydroxides, or the equivalent. Suitablehydroxides include, but are not limited to, NaOH alone or in combinationwith, KOH, LiOH, RbOH, CsOH, Ba(OH)₂, Ca(OH)₂, Al(OH)₃, Ga(OH)₃,Cd(OH)₂, Sr(OH)₂, Zn(OH)₂, La(OH)³, or combinations thereof. Theelectrolyte can be a blend of these hydroxides, preferably NaOH, and/orLiOH without, or with, smaller amounts of the others. The molarconcentration of NaOH is greater than 17%, typically greater than 20%,preferably about 8.4M (25%), and more preferably greater than 14 molar(40%), and most preferably greater than 40% but less than 53%. For thosecases where only ferrate (VI) solution or sodium ferrate(VI) product isdesired, only sodium hydroxide is needed as electrolyte. For otherferrate(VI) products, sodium hydroxide is blended with the appropriatemetal ion hydroxide. For example, for direct production of potassiumferrate(VI), a blend of KOH and NaOH is used. KOH alone is effectiveonly for producing low amounts of ferrate(VI) as rapid coating of theanode with K₂FeO₄ solid occurs. For K₂FeO₄ production, the molar ratioof KOH:NaOH is typically 0.40 or less, and preferably equal to or lessthan 0.25, but greater than 0.02. One preferred electrolyte includesabout 40 to about 45 wt % NaOH and about 3 to about 6 wt % KOH. Duringcontinuous operation the KOH:NaOH ratios are maintained by addition ofadditional electrolyte, water, or concentrates. Removal of productremoves some of the cation which is periodically replenished, normallyby direct additions of concentrate hydroxide solution to the surge tank.Of course, in-line mixing additions also can be performed underautomatic controls, and such make up addition methods are well known tothose skilled in the art of process engineering. Electrolyte density,acid/base titration, AA analysis and/or ion chromatography of cationsare all methods suitable for maintaining electrolyte viability andprocess control.

Ferrate(VI) Production Cell Design and Operation

FIG. 5 shows a typical two-compartment electrolytic vDC powered cell 500according to the present invention. The cell 500 includes a housing 510.Within the housing is an anode 526 and two cathodes 522, 524. There aretwo optional (most preferred) screens 530, 531 located between the anode526 and the cathodes 522, 524. There is preferably just one electrolyteinlet 520. There are preferably two types of electrolyte outlets 524,(anolyte), and 532, 533 (catholyte) for each anode/cathode pair. Theelectrolyte outlets may have a fluid controller not shown in this figurefor controlling the flow of electrolyte out of the cell 500.

The housing 510 can be made of any suitable caustic and oxidantresistant and compatible materials, as is well known to those in theart. For example, metal or fiberglass reinforced plastic with apolypropylene plastic liner, concrete with a rubber liner, polyolefin(polyethylene, polypropylene, polyvinyl chloride, polyvinylidenedifluoride, Viton®, Teflon®, etc.) or other materials could also beused, and combinations thereof.

The anode 526 is made of a material containing iron. Or, the electrolyteis formulated to carry iron or iron ion containing suspended particlesor solution. Suitable anode materials include, but are not limited to,pure iron, cast iron, wrought iron, pig iron and steel. The anode cantake any suitable configuration, including, but not limited to, solidplate (preferred), expanded metal mesh, wire mesh, woven metal cloth,wire, rod, or combinations thereof. Preferably, the anode is a flatplate of iron with minimal amounts of Mn preferably with <0.5% of Mn,and more preferably <0.1% Mn, and still more preferably <0.01% Mn, andmost preferably <0.001% Mn (10 ppm) when the electrolyte contains theiron as particles or solution, then the anodes are selected to benon-dissolving, for example, DSA, Ti, Pt, Pd, Ir and graphite.

The cathode 522, 524 can be made of a variety of materials, including,but not limited to, nickel, titanium, platinum, tin, lead, cadmium,mercury, stainless steel, graphite, alloys thereof, or laminatesthereof. By “laminate”, it is meant one or more layers electroplated orpressed over a substrate, e.g., steel, iron, aluminum, copper, graphite,or plastic. The cathode can have any suitable shape, including, but notlimited to, solid plate, expanded metal mesh, wire mesh, woven metalcloth, wire, rod, or combinations thereof. The cathode is mostpreferably made of nickel-plated steel, nickel-plated iron wires orexpanded metal. A typical cathode would be an expanded metal mild steel,e.g., ST 37, plated with semi-bright nickel.

Optional screens 520, 531 are placed between the anode 526 and cathodes522, 524 and are used to control the flow of electrolyte, the flow ofwhich is schematically depicted by arrows 540. Preferably most of theflow stays in the vicinity of the two faces 527, 528 of anode 526 andaway from cathodes 522, 524. The optional screens 530, 531 are used forflow control to enhance flow within the volume 542 anode side of thescreen. Thus the majority of electrolyte flow will be close to the anodeand will exit at outlet 534. Electrolyte near the cathodes 522, 524 involume 544 on the cathode side of screens 530, 531 will exit via outlets532, 533. Those skilled in the art will appreciate that theelectrochemical cell shown in FIG. 5 can be operated so as to only haveone cathode and one anode (e.g. right hand side of the figure, or tohave a multiplicity of cathodes and anodes as discussed later herein. Inaddition the apparatus of FIG. 5 can have the anodes and cathodesswitched so that the cell 500 now has one cathode in the center and ananode to the left and an anode to the right.

The ratio of the surface area of the anode to the surface area of thecathode, A/C area, is generally at least about three to five, althoughit can be more or less, if desired, including 1:1 or even 0.9:1.0, whichallows a slight overreach of the area of the cathode over the anode toprovide electric field uniformity at the anode surface. More preferredis an A/C area value of 3/1, even more preferred is an A/C area ratio of<1. The advantage of high A/C surface area ratio is that access byferrate(VI), contained in the electrolyte, to the cathode surface isreduced relative to lower A/C ratios. This effect is most desirable froma current efficiency enhancement perspective and it was determined thatmagnetic, black, dendritic crystalline particles are producedefficiently at the cathode during cell operation if the encloseddescribed design and operating precautions are not met and ferrate(VI)is allowed access to the cathode. These magnetic particles are believedto be the magnetite type, Fe₃O₄, and are believed to be formed fromelectrolytic reduction of ferrate(VI) at the cathode, by chemistryequivalent to the following half reaction

3FeO₄ ²⁻+10e ⁻+8H₂O→Fe₃O₄+16OH⁻  (18)

As this reaction requires the negatively charged ferrate(VI) ion todiffuse to the negatively charged surface of the cathode, after beingproduced at the anode, it was possible to invent internal cellconstruction and fluid flow designs to limit this side reaction. Notethat the primary cathodic electrolytic reaction products, OH⁻ and H₂(gas) (Reaction 19) are not so diffusion limited as large amounts ofwater always exist at the cathode, and the cathode is most preferablyconstructed of materials with low hydrogen over potentials (nickel andthe like). Note that the H2 (gas) bubble formation at the cathodedesirably helps limit access of ferrate(VI) ion to the cathode surface.

2H₂O+2e→H₂(g)+2OH⁻  (19)

Hence, the optimal A/C ratio value is set by the lowest of the maximumcathode surface electrical current density possible or the maximum anodecurrent density possible.

The cell can optionally include a screen 30. The screen can be made ofany material which is not rapidly attacked by caustics or oxidizers. Asuitable material is plastic, including, but not limited to,polyolefins, such as polypropylene, fluoropolymers, and polyvinylchloride. The screen will typically have a mesh size of at least about 1mm or less (U.S. sieve mesh) and preferably 0.1 mm or less, but greaterthan 0.01 micron.

In one preferred embodiment, FIG. 5, there is typically one electrolyteinlet 520 per anode. In other embodiments, there could be more than oneelectrolyte inlet. For example, there could be two electrolyte inlets onopposite sides of the cell to provide a uniform distribution ofelectrolyte to the cell. Other arrangements could also be used, ifdesired. In a preferred embodiment, a large number of cells are arrangedin parallel “cell stack” in which the electrolyte is fed to a flowdistributor beneath the anodes and the flow is distributed as per FIG. 5on all electrodes, and where cathodes terminate the ends of the stack.

The electrolyte flows in though the electrolyte inlet 520, divides andflows around the anode 526, and out through the electrolyte outlets 421,533, 534. The screen 530 helps to restrict the flow of electrolyte tothe anode side and hence this stream is referred to as anolyte.Substantially more electrolyte flows past the anode 526 than flows pastthe cathode 522. The ratio of the amount of electrolyte flowing past theanode to the amount of electrolyte flowing past the cathode is typicallyat least 60:40, preferably at least about 80:20, more preferably about90:10, and most preferably about 95:5 or greater. This ratio could alsobe 100:0 (no catholyte flow) but this ratio is not preferred. The amountof the electrolyte flowing past the anode and cathode can be controlledby a fluid controller. Suitable fluid controllers include, but are notlimited to, one or more valves FIG. 13, 1350, or flow restrictions, orweirs in one or both of the electrolyte outlets. When such flowsplitting is performed, it is preferred that the catholyte not berecombined with the anolyte until the surge tank, FIG. 16, 1670.

Scaleable Ferrate(VI) Production Cell:

FIGS. 6 a and 6 b show opposite sides of another embodiment of ascaleable and readily constructed electrochemical cell of the invention600. The electrolyte outlet 610 for the catholyte is shown in FIG. 6 a,while FIG. 6 b shows the electrolyte outlet 620 for the anode. Top ofthe liquid level 621 for cell 600 is shown in both FIGS. 6 a & b. Theweir 622 for the anolyte electrolyte outlet is higher than the weir 612for the catholyte electrolyte outlet 610. Therefore, the catholytecompartment will be assured of sufficient electrolyte to remain full ifthere is liquid present or flow exiting over the anolyte weirs 622. Moreelectrolyte would tend to flow out of catholyte outlet 612 than 662,except the trough 650 collecting the combined flows 640 from 612contains a second adjustable weir, or valve, or other constriction (1350in FIG. 13), that is adjusted such that weirs 612 are kept flooded,forcing the catholyte exit liquid level control 622 to control the flowsfrom 620 to in fact control the flow over weirs 612. As the flow 630exiting from weirs 622 are not restricted as it exits the cell, most ofthe electrolyte flow via associated trough 640 and is forced to exitvery reliably (simple gravity overflow) via weirs 622. Such design canbe easily scaled to enormous production scales, e.g., hundreds and eventhousands of gallons per minute with rugged and reliable design,excellent control, and very low cost, without the need for complex andcostly electronic controls, valves or pumps. Other fluid controllers canoptionally be used, as are well known in the art.

FIGS. 7 and 8 show end-view portions of a typical electrolytic cell 700of the invention. The cell 700 includes tank having a wall 705. The tankcontains electrolyte 707. There is an anode 710 mounted between the tankwalls 705 with close but not touching clearance. The anode 710 isconnected to an electrically connecting crossbar 715 by electricallyconducting hangers 720. The crossbar 715 can be made of copper, iron,stainless steel, carbon, nickel, nickel-plated, Mn steel, nickel-platediron, and the like. The hangers 720 can be made of the same choice ofconductors and covered with a masking agent, such as plater's tape orwax, so that the hangers do not contact or dissolve in the electrolyteduring use. There can be an optional self positioning locator notch 722on each end of the crossbar 715. The locator notches 722 help toposition the crossbar 715 on the tank wall 705 by gravity. There is apositively charged bus bar 725 in contact with the crossbar 715. The busbar 725 connects the anodes to the source of electrical current. The busbar 725 is preferably made of a conductive material such as copper,aluminum, iron and the like. A cathode arrangement is similar to FIG. 7with the bus bar 725 supplying power on the opposite side (dottedcircle).

A screen 830 surrounded by a non-conducting, preferably plastic, frame835 is shown in FIG. 8. The screen porosity being selected is preferablysmaller than most of the H₂ gas bubbles formed in the catholyte. At theapproximate screen mesh size 18 (1 mm), or larger (smaller hole size),selected such that the agitation provided by H₂ gassing at the cathodeis noticeably reduced in agitation of the liquid adjacent the anode, andpreferably reduces anolyte agitation significantly by H₂ gassing, andmost preferably having essentially no transference of H₂ gassing fromnear the cathode to near the anode. The degree to which such agitationis occurring can be monitored quantitatively for process control as itis in proportion to the amount of dendritic magnetic particles producedat the cathode per amount of ferrate(VI) produced. The amount of gaseswhich exit with the separate electrolytes (catholyte and anolyte) mostpreferred is essentially no gas following the anolyte, while substantialgas volume follows the catholyte stream.

Referring to FIG. 9, this figure illustrates a top view for one possibleanode/spacer/cathode combination 900. An anode buss bar 901 is shown onthe left and a cathode buss bar 903 is shown on the right. The viewshows four cathode ends 910, three anode ends 920, and six spacers (thatmay contain optional screens). The ends of the cathode have conductingmembers 915 that extend and lie over the top of the cathode buss bar 903to obtain power. Likewise, the anode ends have conducting members 925that extend over the anode buss bar 901 to obtain power. In this way,the weight of the cathodes 910 and cathodes 920 help provide goodelectrical contact with the buss bars 901, 903. In addition, thecathodes 910 and anodes 920 can easily be lifted out for replacement ormaintenance.

FIG. 9 shows a top view of a typical layout of an electrochemical cellaccording to the present invention. Anodes 710 and cathodes 712 areseparated by screens 711. An anode bus bar 725 connects anodes to 710,and a cathode bus bar 726 connects cathodes 712.

The preferred relative sizes of the anode, cathode, and screen in oneembodiment are shown in FIG. 10. The area of the cathode is at least aslarge as the anode, and preferably slightly larger, by 1-10% than theanode with some cathode extending beyond the anode on all sides. Thescreen, with frame, being approximately as large as the electrodecompartment, does not need to be not tight fitting. FIG. 11 showsexamples of different size electrode combinations for a cell stack ofthe invention. Referring to FIG. 11, some of the parts for a multi-cellelectrochemical cell, 1100, are shown as a side view of the cell stack.Housing 1110, encloses a stack consisting of a first cathode, anelectrolyte (catholyte) compartment, 1150, followed by a first optionaland preferred screen, 1130, followed by a second electrolyte, (anolyte)compartment, 1160, followed by an first anode; this first cell followedby a second cell formed in the reversed order of components to that justlisted, an analyte compartment, 1162: a second screen 1130-2, a secondcatholyte compartment 1150-2, a second cathode, 1120-2 and so one. Inthis view the cathodes are approximately the same length as the screensand the anodes are slightly shorter than the cathodes. However, othersizes and interspatial distance combinations are possible.

Referring now to FIG. 12A, the figure shows an edge on view of cellstack 1600 according to another embodiment of the invention thatincludes a housing 1206, two anodes 1210, three cathodes 1220, and fourscreens 1230 in frames 1232 between each electrode pair. One inlet 1240and distributor plate 1250 is shown supplying electrolyte to the anodecompartments but not the catholyte compartments. The anodes 1210 areslightly shorter than the cathodes 1220. Both sides of the anodes areutilized for ferrate(VI) production in this arrangement. With thisinformation, it is clear that cell stacks containing many more cellsthan the four shown here are possible by repeating this pattern manytimes, even 100-200 cells stacks are possible.

FIG. 12B is a center cutaway view of the same cell stack as in FIG. 12A.This view illustrates the flow of electrolyte through the cells. Arrows1208 generally show flow of electrolyte through the anode compartments1232 and some arrows 1204 show electrolyte flow to the cathodecompartment 1234. Typically, electrolyte flow will be into the anodecompartment and then be divided from there between anode and cathodecompartments. However, in some embodiments electrolyte will initiallyflow into both the anode 1232 and cathode 1234 compartments. Electrolyteeventually makes its way to two anode port areas 1242 and three cathodeport areas 1244.

Referring now to FIGS. 12C and 12D that show catholyte exit cell endpanel 1280 and the anolyte exit cell end panel 1290, respectively.Catholyte flows out from catholyte port areas 1244 through catholyteports 1282. Similarly, anolyte flows out from anolyte port areas 1242through anolyte ports 1292.

Referring now to FIG. 13, this figure is a side view of “L” shaped flowdeflector spacers useful with the invention. The figure shows a housing1302 containing electrolyte 1304. The deflector spacer 1300 fits overthe housing 1302 and allows electrolyte 1304 to selectively exit at port1306 A valve 1350 (or other flow control device) can be used to furtherregulate flow of the electrolyte 1304. In this view±buss bars 1344 and1346 are only shown to indicate their relative position.

FIG. 13 shows an L-shaped flow deflector spacer 1300 located between theelectrodes and screens. The optional flow deflector spacer 1300 has anextension portion 1380 that can be used to close off the electrolyteoutlet 1385 on one side of the tank, while allowing electrolyte to flowout of electrolyte outlet 1390 on the opposite side of the tank. FIG. 13also illustrates an example of the catholyte exit flow restrictor 1350,in this case a simple valve. The spacers 1300, prepared frompolypropylene, for example, can be placed between the cathode 1220 andthe screen 830 and between the screen 830 and the anode 1210, where theflow limiting deflector side of the “L” is positioned in an alternatingpattern as described above.

Referring now to FIG. 14A, this figure depicts a cutaway side view of atypical electrode stack 1400 for one embodiment of the invention. Theunit comprises a housing 1402 containing electrolyte 1404. A firstcathode 1410 id placed along the housing and may have additionalinsulation (not shown) between it and the housing. Next to the housingis an optional screen 1414 followed by an anode 1418 and anotheroptional screen 1422. Spacers 1411, 1415 and 1419 are used to separatethe cathode screen and anode, from each other by a selected distance. Asindicated by arrow 1430 the rest of the space within the housing 1402 istaken up by a plurality electrodes (and optional screens if used) havingthe repeating pattern indicated. The last electrode on the right is acathode 1410. A flow distributor 1450 for electrolyte 1451 is locatednear or at the bottom of the housing. Electrolyte 1404 enters at pipe1455 that enters the housing at port 1453. Electrolyte is distributed byflow holes 1460 and typically flows up as shown by arrows 1461 so as topass between the electrodes above them.

FIG. 14B is a top view of the apparatus 1400 shown in FIG. 14A with theelectrodes (and optional screens) removed. Within housing 1402 iselectrolyte 1404 and flow distributor 1450. Electrolyte enters thedistributor 1450 at pipe a455 flows through the distributor and exits atvarious flow holes to spread throughout the chamber 1465 within thehousing as indicated by arrows 1461.

Referring to FIG. 15, this figure illustrates a typical electrode havingside and bottom spacers. The cutaway view shows a housing containing anelectrode assembly 1500. The assembly 1500 consists of a support havinghangers from which the electrode 1510 is suspended. The electrode 1510is depicted with side spacers 1520 that block electrolyte flow as neededbut allow removal of the electrode from the housing 1502. Button spacers1530 are used to space the electrode from other electrodes or fromoptional screens between electrodes. The bottom of the electrode istypically open and although electrolyte 1540 flow up between theelectrodes.

There can be optional side spacers 1520, 1530 on either the anode or,preferably, the cathode as shown in FIG. 15. The functions of the sidespacers are several. First, they prevent the hanging electrodes fromswinging into each other and shorting out; second, they allow the gapbetween the electrodes to be controlled to very high precision, andhence the electric field uniformity between all adjacent areas of theelectrodes, thus providing uniform anode dissolution rate across theanode surface. Third, the electrode side spacers allow the cell stack tobe assembled quickly using simple clamping tools, rather than having todeal with complex and slow-to-operate machined side grooves in the cellhousing walls. The side spacers 1520, 1530 can be made of a materialwhich is not rapidly attacked by caustics or oxidizers, such as plastics(thermosets and thermoplastics) and rubbers. Suitable plastics include,but are not limited to, polyolefins, such as polypropylene,fluoropolymers, and polyvinyl chloride.

FIG. 16 is a schematic diagram of apparatus, 1600, according to thepresent invention which can be used for the continuous production offerrate(VI). The apparatus included an electrochemical cell, 1610. Thecell stack was operated at a number of anode and cathode arrangements:a) one anode and one cathode, b) two anodes, and two cathodes separatedby an electrically insulated liquid cooling jacket between the cathodes.In both of these arrangements only one face of each electrode is activeduring cell operation. A better arrangement was c) one anode between twocathodes with no external cooling (FIG. 5), where both sides of theanode are active during operation. Preferably, the electrochemical cellincluded two anodes, and three cathodes with no cooling (FIGS. 12A and12B). Note that not needing cooling indicates highly efficientelectrochemical reaction yield as any electrical inefficiencies normallyproduce excessive heat. The anodes and cathodes were separated bypolypropylene screens of various mesh sizes. Electrolyte was heated froma temperature of about 20-25° C. to about 40-45° C. by passing theelectrolyte through a stainless steel coil submerged in a constanttemperature water bath before entering the electrochemical cell, 1620.The anolyte was removed from the electrochemical cell, 1610, and cooledfrom about 40-45° C. to about 20-25° C. or to about 25-35° C., dependingon cooling efficiency using a second heat exchanger, 1630. Cooling to20-25° C. is preferred for highest yields (slower product decompositionrate). The anolyte was then sent to a crystallizer, 1640. The output ofthe crystallizer was pumped, 1660, to a solid/liquid separator, 1660,which was a 10μ filter or spiral wound porous filter, depending upon thetest performed. The solid needle-shaped ferrate(VI) product wascollected using this filter, or more often, by manually filtering theanolyte in portions during operation, each time returning the filtrateback to the crystallizer surge tank, 1640, via manual line, 1661. Thesolid product was collected, 1662. An alternate mode of operation, thefiltrate was sent to another surge tank 1670, where makeup hydroxidesand/or water were added as necessary via line 1671. The catholyte wassent from the electrochemical cell, 1610 to the surge tank, 1670, vialine, 1620. The electrolyte was then recycled from the surge tank, 1670,back to the electrochemical cell 1610 via pump, 1680, through optionalheat exchange, 1620. Power input to the cell is via line, 1613. Thisapparatus with separate and combined crystallizer/filtrate surge tankswas used for Examples One through Four.

The following examples are illustrative of the invention and are notmeant to limit the scope of the invention in any way.

Example 1

This example illustrates the use of batch filtration and the ferrate(VI)production apparatus, 1600 (See FIG. 16).

Four runs were made. Ferrate(VI) crystals were periodically harvestedduring each run using a batch filtration process (evacuated looselycovered Büchner funnel/filtration flask open to the room air pressure).In this mode of operation valve 266 by-pass line was open (10μ filtervalve out) and valve 267 was opened only long enough to gather thesample. A known amount of electrolyte (normally 1 to 2 gallons) waswithdrawn from the crystallization tank via valve 267 and vacuumfiltered manually to obtain a cake containing potassium ferrate(VI)micro-fiber crystals with adsorbed electrolyte (a solution of water,NaOH, and KOH), and any Fe-containing by-products, especially ferrichydroxide or a magnetic chunky black crystalline product consistent withmagnetite. The filtrate was recycled back to the cell. Although this wasonly a batch or semi-continuous separation operation, it wassufficiently continuous on the time scale of the process to be effectivein keeping the ferrate(VI) production rate and efficiency high andpreventing ferrate reduction at the cathode, or by bimoleculardecomposition reaction interaction, or by some other means. By keepingthe potassium ferrate(VI) concentration low, the reaction leading tosolid ferrate(VI) salt is still achieved while limiting the reduction ofdissolved ferrate(VI) ions to other undesired iron species.

It was found that a sudden rise in current efficiency Ieff at 2750-2800minutes in the above described test can be attributed to the use of alower total electrolyte volume/anode area test.

External, batch centrifuging of the electrolyte samples provided asimple method of removing the desired product from the electrolytewithout significantly changing its chemical and physical properties and,more importantly, without the need of any post-processing as is requiredwith filtration (i.e., leaching and re-crystallization of the ferratefrom the wound filter media). Also, it should be noted that with anexternal batch filtration process, the ferrate concentration in theelectrolyte builds up and comes into repeated contact with the cathode,resulting in reduction decomposition giving loss of product andtherefore lower Ieff values than in continuous process.

Example 2

This example illustrates the use of a continuous centrifuge for recoveryof ferrate product crystals.

As Example 1 demonstrated, removal of ferrate(VI) salt helps to achievehigh production rates. (FIG. 18 proves this relationship mostconclusively.) Therefore, an in-line centrifuge was tested forcontinuous crystalline K₂FeO₄ product removal. The in-line separationwith a centrifuge was very effective in removing ferrate(VI) crystalsfrom the electrolyte, as seen in FIG. 17. As observed in tests accordingto the invention, crystallization occurs at ferrate concentrations aboveabout 4 mM at this wt. % KOH concentration and at about 25° C. Aftereach centrifuging procedure, the concentration was decreased to thislevel, verifying this solubility. The solubility was also verified byabsence of particulates in the centrifugate by optical microscopy (OM).This observation indicates the successful and essentially completerecovery of ferrate solids from solution. The product that resulted wastypically about 5 wt % ferrate(VI) and physically behaved as a pourable,but very thick material. After the product was pressed at 110 psi, whichremoved additional electrolyte from the fine fiber cake, the K₂FeO₄content increased to 20 wt. % or more. The solubility of potassiumferrate in the electrolyte at these conditions was measured as 4-5 mM(filtrate and centrifugate supernatant concentration) with a temperaturevariation of ±5° C.

Example 3 Example 3a

Fully filtered 1.5 v-12 volt, from 1 A-400 A total current, DC power (noapparent ripple) and commercially available 6 @ Hz AC line power are byfar the most readily available electrical power sources. Hence thesepower sources were evaluated for ferrate (VI) production in batch andcontinuous-flow electrolyte cells. In both cases both power types werefound to either produce no ferrate (VI) at all, or initially a faintamount, followed by no additive ferrate, as evidenced by the lack offormation of a purple, or any, color to the electrolyte. Hence highconcentrations of ferrate, and hence high ferrate production rates, didnot seem possible by this route.

Example 3b

Using the cells described above but powered by low-cost and availablerectified by unfiltered DC power, nominal 20 Hz, as is available fromautomobile battery charges, it was discovered that large amounts offerrate(VI) are formed rapidly in both batch and flow-throughelectrolyte cell configurations. Using an oscilloscope, it wasdetermined that such power sources do not have a current reversal (AC)component.

Example 3c

By coupling an AC transformer with a rectified and filtered DC powersource (up to 55 A and 30 volts), it was determined that any currentreversal (AC component) was a detriment to ferrate (VI) productions byformations of nonferrate (VI) soluble species and particulates.

Example 3d

Hence, based on the results of Example 3a, 3b, and 3c, it was determinedthat the required power supply for ferrate (VI) production needs to bevariable DC (vDC) only at about the minimum of 20-22 A/[see preferableanode batch data]_cm².

This example shows that the power source type is critically important tohigh ferrate production rates per unit area of anode. In addition tothis increased production rate for large-scale commercial production,continuous flow-through cell operation is most preferred. The followingexamples illustrate how this combination was accomplished while avoidingthe decomposition and side chemical reactions characteristic of suchenergetic materials as is ferrate (VI).

FIG. 17 illustrates the continuous production of ferrate(VI) for a longperiod. The ferrate(VI) accumulated to about 11.2 mM prior to Na₂FeO₄crystallization. KOH was then added to induce K₂FeO₄ crystallization,which produced a thicker microcrystalline fiber but with a lower aspectratio, about 10-20. In this run, the ferrate(VI) was allowed toaccumulate significantly between harvests, giving the concentrationprofits a saw toothed shape with run time. The fact that the slope ofthe saw tooth was about the same filter/growth cycle indicates a stableprocess operation for over 5500 min. Both centrifugations and filtrationwere found to be effective and efficient for separation of product fromthe electrolyte. Importantly, the steady and repeatable level ofperformance indicates that the electrolyte is stable and is reliablyrecycled using the conditions of the invention. That the ferrate(VI)concentration dropped to nearly the same value after each filtration,suggests that the solubility of ferrate at these process conditions isabout 4.5-5.5 mM.

This example illustrates the power supply variable DC waveform and waveproperties required for continuous, high ferrate(VI) production ratecell operations. In this first test, a modified sine wave was used. Innearly all cases, the top portion of the waveform was found to bechopped, or flattened. This voltage is defined herein as Vmax. While notwishing to be bound by theory, this observation is interpreted to be dueto reaching the Fe(0)→Fe(VI) oxidation potential, thereby effectivelyreducing internal resistance (increased electron flow by chemicalreaction). The oxidation of Fe(0) to Fe(VI) occurs at the higher voltageas evidenced by purple color formation. By modifying the waveform inthis manner to force the voltage to remain at this high value for asignificant part of the cycle, but also reducing the voltageperiodically to provide the required variable DC, the production ofFe(VI) may be maximized. This variable DC effect was observed duringtests according to the invention.

A modified square wave was also tested. This waveform, though appliedsquare, did not remain so across the cell at the highest frequenciesbecause the amplifier was not capable of the necessary fast responsetime due to the amperage required. The voltage sweep time from theminimum to the maximum voltage was therefore substantial, resulting in awaveform more resembling a sine wave chopped on the top and bottom. Inaddition, the Vmax→Vmin slew shape appeared to consist of two steps, thefirst an apparent chemical potential generated within the cell, and thesecond, a change in the Vanion setting. These two waveforms, and asaw-toothed waveform, allowed testing the effects of Vmin duty cycle. Inthis manner, it was determined that a modified square waveform resultedin higher current efficiencies for ferrate(VI) production, i.e., theVmin duty cycle needs to be approximately long enough to allow theobserved “decay” in voltage due to a second, not electrochemical, redox(oxidation-reduction) reaction. Not wishing to be bound by theory, it isapparent that this critical second reaction phase (the first beingdissolution of anode at Vmax), corresponds to disproportionation ofreactive intermediates of iron, forming more ferrate(VI), which issoluble and so diffused away from the surface, this thinning the oxidefilm there, preventing the buildup of an electrically resistivepassivating film, which otherwise prevents ferrate (VI) production.

Example 4

This example studied the effect of KOH concentration and addition time.

During tests of the invention, it was observed that stable potassiumferrate(VI) salt crystals can be produced directly during ferrate(VI)production and obtained in good yield using a blend of NaOH and KOH aselectrolyte. However, it was also found that high KOH concentrationcauses a dramatic reduction in cell current efficiency compared to NaOHalone. Therefore, there is an optimal KOH concentration effectiveconcentration range. Preferably, KOH is added after electrolysis wasbegun to initiate strong ferrate(VI) production on power up.

After startup, an initially high current efficiency was found todecrease over time. In most of the screening test runs, the currentefficiency began dropping before the KOH was added, indicating that theferrate(VI) production rate was hindered by some parameter other than K⁺concentration. The screening testing also revealed that isolated sodiumferrate(VI) salt is possible and that it is unstable benefitingsubstantially from anhydrous refrigerated storage conditions, or shouldbe used within days of production. It is also so reactive that theleach/recrystallization procedure to convert it to K₂FeO₄ proceeds withhigh yield loss. A build up in ferrate(VI) concentration in NaOHsolution will lead to decomposition and lower current efficiencies.

The KOH concentration was varied during these tests utilizing 2, 4, and8 wt % KOH in the electrolyte.

Example 5 Screening Testing: Experimental Data Summary

Four runs were carried out with a similar cell configuration;polyvinylidene fluoride (PVDF) construction using one iron anodesandwiched between two nickel cathodes providing a total of 866 cm² ofanode surface area. No Nafion membrane was used. No screen flow divider(see below) was used, but the flow baffles were, allowing low cellvoltage and the removal of the second (cathodic) electrolyte feed (FIG.19) stream and associated plumbing, tank, and pump (FIG. 16). A singleelectrolyte solution also simplifies the process. This solution waspumped at about 1.5-1.8 gpm through the cell, where it contacted bothsides of the iron anode in a parallel flow pattern. A flow distributorin the base of the cell provided uniform flow rate to both sides of theanode. Dual power contacts helped provide uniform (top and bottom)electrical current to all the electrodes. To avoid over dilution ofelectrolyte, the experiments were begun by pumping about 10 L of 45 wt %NaOH solution through the system for approximately 30 min. The cell wasthen operated for a known period, approximately 1000 min. before addingappropriate amounts of KOH solution. This allowed sufficient time forsodium ferrate(VI), Na₂FeO₄ of H₂O crystals, to form. The period, untilKOH addition, was reduced substantially later, which avoids Na₂FeO₄crystallization and goes directly to K₂FeO₄ product. The resultingelectrolyte volume was between 14-18 L with NaOH concentration of about32 wt % and KOH concentration of 2, 4, or 8 wt %. Filtration wasperformed by tapping a volume of electrolyte (normally 1 gallon) fromthe crystallization tank discharge valve and performing a 1 atm. vacuumBuchner funnel filtration.

During this run, an in-line centrifuge (contrafuge) was testedsuccessfully for high-yielding solids separation at continuousconditions (FIG. 20). When centrifuging, a peristaltic pump was used totransfer electrolyte from the crystallization tank to the centrifuge ata flow rate of 100 mL/min. The centrifuge was operated at various spinspeeds over the whole range available to the device (6000-10,000 rpm).Separation was excellent over this entire range. Therefore, the product(and by-product magnetite crystals and “sea urchins” product crystals)can be recovered easily by centrifugation. It is estimated that even1/100 these speeds, i.e. 60 to 100 rpm could be effective, but 1/60^(th)this speed (600 to 1000 rpm) would be preferred, especially at largescale. For example, after 1 hr. of centrifuging, the separation processwas stopped, and the product examined. The product from the centrifugewas typically about 5 wt % ferrate. This was increased to about 20 wt %by pressure filtering the cake at 100-130 psi, thereby making a solidwafer “tablet”. Such tablets were easily handled and compact, yet stillreadily dissolved in water and, hence, represents an excellentcommercializable product form to use for water treatment or other uses.Throughout the test, ferrate(VI) concentration in solution wasdetermined by the UV-Vis test described below.

The experimental conditions and initial current efficiencies for thetests are included in Table 1. In each test, the waveform was alteredover a limited range to determine any effect on ferrate(VI) production.

Although the waveform was varied during each experiment, the waveformand voltage characteristics listed here are the initial settings onlyand are valid for the listed current efficiencies. These are notoptimized parameter values.

-   -   T    -   T

TABLE 1 Experimental Conditions and Results for Four Tests Freq VmaxVmin Initial Current Run 1 Run Waveform (Hz) (V) (V) Efficiency (%) 1Square 1.0 3.40 1.60 10.8 2 Sine 1.0 3.00 1.32 9.2 3 Sine (flat top) 2.03.08 1.72 10.0 4 Sine (flat top) 2.5 2.88 1.72 13.4

The ferrate concentrations measured during this test are presented inTable 1. The initial current efficiency was about 10% in NaOH solution.Although the Vmax was altered at 375 min. run time from 3.40 V to 3.00V, this change did not result in any noticeable change in ferrateproduction rate, indicating that the same production rate is possible atlower power. It was then determined that the voltage could be reducedeven further, to about 2.7 volts. It should be remembered that absolutevoltage values are a function of cell design, temperature, electrolyteconductivity, and anode-cathode spacing.

Run 2

This test utilized four different waveforms as described in Table 2. Theaddition of KOH to the system was done in conjunction with the change inwaveform from a sine (single point Vmax) to a sine with flat top.

TABLE 2 Waveforms Used in the Run 2 Tests Time On Wave Stream (min)Waveform Description 1   0-1161 1 Hz sine, Vmin = 1.32 V, Vmax = 3 V,Imax = 54 A, I min = 0.5 A 2 1161-2885 2 Hz sine with flat top 220 ms,Imax = 54 A, I min = 14 A 3 2885-3850 2 Hz square with flat top 240 ms,20 ms slew up and down, Imax = 51 A, Imin = 3 A 4 3850-6792 2 Hz squarewith flat top 400 ms, 100 ms slew up and down combined, Imax = 55 A, Imin = 33 A

It is known that in some large industrial scale electrochemical process,for example, persulfate or hydrogen peroxide, the ratio of anolytevolume to anode surface area is a key design parameter. Therefore, theeffect of electrolyte volume was examined. During the last filteringstep (about 6650 min on stream), the electrolyte was decreased from 17 Lto 19 L. This decrease in volume resulted in an increase in bothproduction rate and current efficiency by about six fold.

It was also observed that decreasing the electrolyte volume by 17 L to 9L immediately resulted in a substantial increase in ferrate(VI)concentration. This observation suggests that decreasing the ratio ofthe volume of electrolyte/anode area (Velect/Aanode) decreasesferrate(VI) production, all else remaining the same.

These tests were qualitative in nature and do not provide optimized %KOH, % NaOH, or K/Na ratio levels. It is well known in the art that suchoptimization is determined using systematic, preferably statisticallydesigned, tests so that these parameters are optimized (set points andcontrol windows) in conjunction with the other process parameters,including continuous product removal, by-product prevention,heating/cooling effects, and the like.

Run 3

During this test, separation of solids was performed by frequentfiltration only; the centrifuge was not used. The waveforms used in thistest are described in Table 3. A sine wave with a flat top was usedthroughout most of this test.

TABLE 3 Waveforms Used for Run Time on Stream (min) Waveform Description0-47 2 Hz sine wave, unmodified, Vmax = 3.08 V, Vmin = 1.72 V, Imax = 45A, I min = 8 A 47-922 2 Hz sine wave, modified with flat top (170 ms) at50 A, Vmax = 3.08 V, Vmin = 1.72 V, Imax = 50 A, I min = 8 A 922-18192.5 Hz sine wave, modified with flat top (160 ms) at 48.5 A, Vmax = 2.88V Vmin = 1.76 V Imax = 48.5 A I min = 13.5 A 1819-2385  1 Hz square with500 ms top at V = 3.08 V, Imax = 50 A, Vmin = 1.72 V, I min = 1.5 A, 80ms bottom trough

FIG. 21 summarizes the test results using the described cell of theinvention with periodic centrifugation or filtering for the four waveforms, voltages, current, and frequency combinations of Table 3. Theplot provides the ferrate(VI) concentration in mM versus run time overabout 7000 minutes, or about 30 times longer than reported in the priorart. All waveforms were found effective for ferrate(VI) production. The1 Hz wave form produces ferrate(VI) at a faster rate than the 2 and 2.5Hz settings. This advantage was attributed to the extended t₃ value atthe 1 Hz setting versus the 2 and 2.5 Hz setting. Note that the sharprise during the first 1000 min is typical and represents the startupcondition in which ferrate(VI) concentration builds to supersaturationand then forms microcrystals, in this case at about 900 min into therun. Once microcrystalline product forms, as indicated from periodicoptical microscopy of the electrolyte, then solid/liquid separationoperations were performed at the indicated times. The total ferrate(VI)in solution and in suspension drops as expected due to this productharvesting. The maximum ferrate(VI) concentration reached beforecrystallization occurred was 9 mM, and this is reduced to about 1.8 mMwith frequent filtering. Hence 1.8 mM is the approximate solubility ofK₂FeO₄ at production process conditions.

Run 4

This run used 1.92 wt % KOH which was added 80 min. after start-up. Atthis time, the waveform was also changed from a sine wave to a sine wavewith a flat top. Table 4 describes the waveforms used for this run.

TABLE 4 Waveforms Used in Run 4 Run Time (min) Waveform Description 0-75 sine wave, 2.5 Hz I min = 3 A, Imax = 33 A, Vmax = 2.32 V, Vmin =1.48 V  75-3440 sine wave 2.5 Hz, with flat top 120 ms, duty cycle sineshaped bottom, I min = 12 A, Imax = 47 A, Vmax = 2.88 V, Vmin = 1.72 V3441-4211 sine wave 5 Hz, with flat top 100 ms, sine shaped bottom, Imin = 12.5 A, Imax = 17.5 A, Vmax = 2.44 V, Vmin = 1.80 V

For most of the run, a sine wave with a flat top was used. This data wasinterpreted to indicate that continuous product removal helps to preventproduct reduction to by-products in the cell when the electrolyte isrecirculated. This result was later confirmed by additional testing.Near the end of the test run, the frequency of the wave was increased,and the current was decreased. The lower current being supplied to thecell resulted in a lower ferrate production rate. This result verifiedearlier results that indicated that ferrate production rate isproportional to current if other parameters are held constant (at leastat this general current density).

The concentration of ferrate(VI) in solution versus time in minutes ispresented in FIG. 22. As seen in this figure, the concentration reachedhigh levels during the initial stage of the run, even after KOH (toabout 2 wt %) had been added. All of the various separation processes(centrifuge, filtration, pressure filtration) used in this run workedwell in removing the solid ferrate from the electrolyte. After 1500 minon stream, the filtration and centrifuging resulted in nearly completeremoval of solids as indicated by the resulting ferrate concentration ofabout 4 mM in the electrolyte at these conditions. This residual is nearthe saturation point for ferrate(VI) in the NaOH/KOH electrolyte.However, the sharp increases in ferrate(VI) concentration and rate atthe beginning and middle of the run indicated that a continuous solidsseparation process should be employed to avoid losing the product at thecathode. Frequent separations resulted in good agreement between the 505and 785 nm spectral peaks in the UV-VIS spectra of the ferrate samples(using the procedure is described elsewhere in this application),indicating a high purity of Fe(VI) in the samples as ferrate(VI) ion andlow levels of by-products (magnetite and FeOOH colloids).

FIG. 22 illustrates that KOH can be added early in the operation of theferrate(VI) production of the invention, hence not making Na₂FeO₄crystals first, for example as was done for the data of FIG. 17. Thisdata again shows stable, long term operation, to over 4000 min, of theprocess of the invention. The ducal lines of the plot provideferrate(VI) analysis results for the two diagnostic visible wavelengthsfor ferrate(VI) (see example 9). Hence the closeness of the curvesindicates that approximately pure, by-product free ferrate(VI) wasproduced over the entire run, with only moderate indications ofparticulate impurities present after running the periods without productfiltrations.

FIG. 23 demonstrates long term, stable and continuous ferrate(VI)product production using a cell of the invention where sodiumferrate(VI), Na₂FeO₄, product needles are made, and where potassiumferrate(VI), K₂FeO₄, product needles are made. As previously described,the closeness of the two lines indicates high ferrate(VI) productpurity. As most filtrates show a similar concentration (the minimumvalues in these plots) of about 6 mM, this is concluded to be thesolubility of potassium ferrate(VI) in the electrolyte at theseexperimental conditions. This data illustrates that potassiumferrate(VI) product is effectively crystallized at low KOHconcentrations and at high KOH/NaOH ratios. This data furtherillustrates that high ferrate(VI) concentrations, in this case about17-18 mM, are possible with the process.

Example 6

This example was used to validate the findings of previous examples. Theresults from previous tests indicated that following modifications andparameters were likely to improve electrical current efficiency, productpurity, and continuous operation.

1. Decreased electrolyte volume/anode surface area ratio. This parameterwas adjusted using two methods. First, a second iron anode was installedin a cell, increasing the total surface area from 866 cm² to 1732 cm².Secondly, the total volume of electrolyte in the crystallizer/surge tankwas decreased.2. Waveform. A waveform with both a flat top and bottom was employed(square wave). The duty cycle of the square wave was adjusted to allowsufficient time for Vmin to stabilize, indicating completed secondaryreactions, then reset to Vmax. A possible mechanism for this secondaryreaction effect is that the tailing out and flattening of the waveformat Vmin limits the buildup of passivating film thickness, critical tocontinuous production, and perhaps reaction intermediates formed at thehigher voltage.3. KOH addition point. KOH was introduced a short time afterelectrolysis is began. Also, the effect of low KOH concentration was tobe verified.4. Screen. A thin open polypropylene screen (not a membrane) was used toseparate the anode and cathode compartments. The exiting electrolyteswere kept apart, and an almost closed valve was added to the exitingcatholyte line to reduce flow across the cathode. This was done tominimize/prevent hydrogen from the cathode from contacting theferrate(VI) produced in the anodic compartment, and to limit FeO₄=accessto the cathode. By limiting the cathodic reduction of ferrate(VI) in thesystem, the overall yield should increase.5. Reduced lower current density. Current density was reduced byincreasing (doubling) the anode surface area in the electrolytic cell.Current density could be decreased further by reducing the total currentthrough the cell, but this option was not tried here in order to keepferrate(VI) production rate as high a possible. Literature dataindicates that current yield reaches a maximum around 3-4 mA/cm² forshort run times. The tests according to the invention were typically runaround 57 mA/cm² (with one experiment at about 20 mA/cm²), whereliterature data indicates a low in current yield. The actual goodproduction rates of this invention may suggest that the short-runcurrent densities from the literature are not preferred. Instead,maximum current per cell volume is preferred, so long as the currentdensity used is still on the linear current vs. ferrate production ratecurve.6. Temperature Control. Literature data indicates that highertemperature (about 40-50° C.) results in increased current efficiencies,approximately three times greater than those obtained at 20° C. However,previous work and the same literature indicate that ferrate(VI)decomposes at these elevated temperatures, resulting in a high totaliron to iron(VI) ratio. As a result, most of the previous experimentalwork was done at 25° C. Here, the electrolyte was heated to as close to50° C. as the equipment allowed before entering the cell, and thenimmediately cooled back down to as close to 20° C. as the equipmentwould allow on exiting. The idea was to maximize Fe(VI) production ratewhile limiting ferrate(VI) decomposition, and to facilitatecrystallization yield. These objectives were met (see below).

The tests were performed using a modified cell configuration,alternating two iron anodes placed between three nickel cathodes. Apolypropylene plastic screen was placed between the anodes and cathodesto inhibit contact between ferrate and the cathodes or H₂ gas bubbles.In the novel design, a single electrolyte solution was pumped throughthe system and contacted both the anodes and cathodes. As the analyteand catholyte liquid levels are equal, and the analyte is free to exit,while the catholyte exit flow is restricted by the exit valve,substantially more electrolyte flow passes over the anode from thecathode. This flow difference is readily observed at cell startup, wherethe purple color of ferrate is observed for some time in the analytebefore it appears in the catholyte. This flow differential decreasesferrate contact with the cathode proportionately and, therefore, ferratelosses by this route.

The experiments were begun by pumping 10.8 L of 43 wt % NaOH solutionthrough the system for approximately 60 min., while the cell isenergized, before adding appropriate amounts of KOH solution. Theresulting electrolyte volume was 11.0 L. Throughout the test run, theNaOH concentration was 42-45 wt % and the KOH concentration was 0.8-1.3wt %. The values can be verified by density, acid/base titration, andthe AA analyses.

The electrolyte was heated to 43° C. before entering the electrolyzerand cooled to 37° C. in the surge tank. Although the ideal temperatureswould be 50° C. and 20° C., respectively, these values were notattainable with the plastic heat exchange tubing for cooling. Theplastic tubing was used to avoid ferrate(VI) attack on the steel,resulting in contamination/destabilization. Stainless steel (alloy 316)worked well for heating, while 304 stainless steel was corroded, if usedfor the cooling heat exchanger. As ferrate and caustic is always in theelectrolyte at both sites, and cooling is more stabilizing than heatingwith respect to materials' resistance to oxidizers and caustic. It isconcluded that 316 and greater SS is compatible with the ferrateprocesses of the invention, while 304 SS and lower is not. Therefore,the use of stainless steel was demonstrated to be viable in continuousferrate(VI) production on arterials of construction. Stainless steel of316 and higher can be used for various parts of the equipment including,but not limited to, heat exchanger tubing, electrolyte fluid piping, andsolid/liquid separation hardware (sieves, filters, centrifuges,crystallizers, tankage, hydrocyclones, and the like). Suitable stainlesssteels include 316 stainless steel, as well as higher alloys ofstainless steel and nickel. 304 stainless steel is not suitable forthese applications because it is attacked and corroded by theferrate-containing electrolyte. Such corrosion also leads to manganesecontamination of the ferrate product, as stainless steels contain >0.3%Mn.

An in-line hydrocyclone was used to continuously separate solids fromthe electrolyte. The use of the hydrocyclone required a larger volume ofelectrolyte. Therefore, additional electrolyte was added to the systemat 390 minutes, bringing the total volume to 17.8 L. The in-linecentrifuge was run continuously to remove solids from the electrolytestream. The resulting centrifugate was about 1% solids, as needed toseparate the product from the electrolyte so that the electrolyte couldimmediately recycled. The 1% slurry was filtered off line to produce ahigh percent active product. The filtrate was added back to the processas viable electrolyte. It was later discovered that the cyclone wasplugged with solids, indicating rapid and good solid/liquid separationwas occurring. As too much product was being held back, it is well knownin the art that this blockage would be prevented by weir heightreduction to allow the slurry to exit the unit without excessive caking.Such weir size determination is within the skill of the art.

With the hydrocyclone off line, filtration was performed by tapping 2-4L volume of electrolyte from the pump exit line and performing a vacuumfiltration similar to Run 1 of Example 5, but as continuously aspossible. The product from the filtration step was typically anexcellent 9 wt % potassium ferrate(VI). Earlier pressure filtrationtests indicated that the percent active product was readily increasedto >20% by this means. The retention of the electrolyte by the productis believed to be due to the high surface area of needle-like microcrystals, which are squeezed when under pressure (e.g., compression),which reduces inter-crystal void volume, which rids the crystals of aproportionate amount of the viscous electrolyte. Because such crystalsoften “spring back,” the liquid needs to be removed while the crystalsare still under pressure, i.e., in a compressed state. This effect helpsproduce a high percent active product, >10% and often >20%. Withoptimization of pressure filtration, it is believed that this valuecould be raised to 30 to 70% and even higher if larger crystals could beformed by crystallizer optimization.).

The electrolyte volume was decreased to 9.2 L at 1654 min to validatethe observation made in earlier tests that low electrolyte volumeresults in an increased current efficiency. Throughout the run,ferrate(VI) concentration in solution was determined by UV-VIS asdescribed below.

The waveform used in this experiment was a square wave of 1 Hz. Themaximum and minimum voltages were 2.20 and 1.26V, and the maximum andminimum current were 56 and 0.4 A, respectively. The power level wasabout 123 watts, which is desirably low. The waveform was captured withan oscilloscope. Although the waveform of the power supply was a squarewave, as noted previously, tailing was observed during the down sweep,but little on the up sweep. This approximately exponential voltage dropfrom Vmax→Vmin is interpreted by us as an indication that someoxidation/reduction (“redox”) chemical change is occurring during thedown sweep, i.e., that the cell is behaving as an electrochemical cellduring this Vmax→Vmin transition period and as an electrolytic cellduring the Vmax plateau region. While not wishing to be bound by theory,this result is tentatively interpreted to be an indication that theoxide film might be reacting (thinning) in the downward sweep byequilibration to produce soluble Fe forms (e.g., Fe(V), Fe(VI), orFe(II)). This chemistry would thin the oxide layer and prevent itsthickening (this is a critical feature of the invention, as layerthickening would lead to passivation). The resulting cell currentdensity was 32 mA/cm² at Vmax, i.e., for greater than the optimal valuesindicated by the short tests described in the literature.

Using these settings for the process parameters, the current efficiencyand production rate were observed to increase substantially overprevious trials, even after the startup period. In addition, the currentefficiency was essentially constant. From the total amount offerrate(VI) produced, which is shown in FIG. 17, the production rateduring this entire 4500 min. experiment was calculated as an excellent64.1 g/day (0.14 lb/day). Previous tests had high current efficienciesat startup, but not this high. Therefore, the combination of waveform,cell configuration, continuous product removal, temperature control, lowelectrolyte volume, and low wt % KOH, used simultaneously in the presenttest, resulted in this high current efficiency (at least about 28%) thatwas sustained throughout the run. This level behavior indicates thatthese parameters were under control and that the parameters needed tocontrol current efficiency and the entire process was being controlledover long production times.

FIG. 18 clearly illustrates the value of continuous harvesting offerrate(VI) product when operating the cell of the invention. Althoughthe process exhibited a high degree of robustness by repeatedlyoperating overnight unattended, the net ferrate(VI) production rate wasfound to be far better during periods where continuous harvesting wasperformed. Specifically potassium ferrate(VI) production rates of 108mg/min, 77.3 mg/min, 81.1 mg/min and 60.7 mg/min K₂FeO₄ were produced oneach of four consecutive days. This data again illustrates the viabilityof a long continuous operation.

The fluctuations in the isolated ferrate product (FIG. 18) during therun provide critical additional information. In particular, duringovernight operations where solids separation was not performed, theamount of ferrate produced decreased. However, for example, from2500-3000 minutes, when filtration was performed at a high frequency,the ferrate production values (slope over this time interval) were quitestable (81.1 mg isolated K₂FeO₄/min), indicating the importance ofsolids separation in production yield and, therefore, since the powerwas constant, current efficiency. This is further evidenced where the785 and 505 nm UV/VIS peaks for the electrolyte samples are compared.Any deviation between these two measurements indicates the presence ofunwanted iron by-products. Mid-way through the run, when filtration wasperformed frequently and the electrolyte volume was low, the agreementbetween the two measurements was good, which indicates a lowconcentration of other iron species.

After the process was shut down, the cell was disassembled, and theelectrodes were examined. An amount of black material, apparentlydendritic magnetite, had accumulated loosely on the bottom portions ofthe cathodes. In previous experiments, a thin layer of dendriticmaterial was observed on the cathodes after shutdown, with a concurrentamount of material flowing in the electrolyte solution. In this run, thedendritic material was not observed in the solution and appeared to belimited to being trapped in the cathode compartments, because of theslow flow rate in the cathodic compartment relative to the anodiccompartment made possible by the valve in the catholyte exit line andthe presence of the screens in the cell. The presence of this materialon the cathodes provides evidence that magnetite is formed in thecathode from ferrate(VI). This observation validates the strategy ofpreventing ferrate species from entering the cathode compartment and thevalue of using the slow catholyte flow/screen technique to control itsformation. It was found that these solids could be removed from thecathodic compartment by periodically opening the catholyte valve for ashort time to increase catholyte flow rate to flush the magnetite to thefilter. The magnetite can be separated as a co-product.

The effectiveness of the hydrocyclone indicates that usingcentrifugation in a batch or continuous mode is effective for productrecovery from the electrolyte, even at low G, centrifugal, force, forseparating ferrate(VI) crystals from the electrolyte rapidly andcompletely, while simultaneously regenerating the electrolyte forrecycle. Batch filtration of the centrifuge cake or slurry forcontinuous processing was used to isolate a high percent active product,up to >20%. The results show that the centrifuge and hydrocyclone wereextremely effective in removing ferrate solids from the electrolyte.This continuous removal of ferrate(VI) solids from the system helps toprevent the decomposition of product caused by recirculating it to thecatholyte and forming reduced iron species (magnetite and ferricoxyhydroxide colloid). These by-products could also be removed from theelectrolyte by occasional filtration if needed, but preferably, as theirformation represents ferrate production yield loss, their formation isto be avoided.

The results of the ferrate production rate enhancements confirmed thatthe cell efficiency increased when the total electrolyte volume wasdecreased, indicating the electrolyte volume/anode area should becontrolled. This result was later verified further.

When the power supply waveform was changed to a flat Vmin, versus asingle-point Vmin (sine wave to square wave), the electrical currentefficiency (% Ieff or CE) increased markedly. This efficiencyimprovement was verified during additional testing as well. Using awaveform with a flat top and bottom resulted in good currentefficiencies. These results were interpreted mechanistically asindicating that Vmax builds ferrate(VI) and oxide film from the ironmetal, while Vmin depletes the oxide film by forming ferrate(VI), andperhaps ferrate(V), thereby preventing the buildup of a passivatinglayer of oxide (which is produced too fast when a non-variable DCcurrent is used). The testing also suggested that good ferrateproduction rates could be obtained with low (2 wt %) KOH concentration.

Example 7

The findings described above from Example 6 were incorporated into thistest. The objective was to validate the findings from the earlier runsand to demonstrate that high ferrate production rates and currentefficiency can be obtained and held during a continuous run. Theparameters included low electrolyte volume/anode area, continuous solidsseparation (by centrifugation or filtration), decreased current density(by doubling anode surface area), separation of anode and cathodecompartments using a screen instead of a membrane, and use of a squarewaveform (i.e. Vmax and Vmin both flat), and low wt % KOH.

These adjustments resulted in high current efficiency that was sustainedthroughout the entire run. The efficiency was at least 28% up to about62% over 4438 minutes (>3 days) at 2.20V. These current efficienciesrepresent an economically viable process. For example, furtherimprovement in the current efficiency may be obtained by 24/7 solidsseparation (using an in-line continuous centrifuge hydrocyclone, orfilter that is sized for the production cell), lower crystallizertemperature and optimized current density.

Another important finding is that metals, such as nickel and stainlesssteel can be used at certain points in the process (cathode, piping,filter, internals, centrifuge internals, hydrocyclone internals, andheat exchanger elements, etc.) without apparent detriment to thestability or contamination of the ferrate(VI) product. Also, the amountof magnetite produced was found to be a very low level using the newdesign for the cell internals.

Flow distributors optionally may be used in the anolyte and/or catholytecompartments. Flow distributors force better contact between reactionintermediates in solution, namely Fe(IV), Fe(V), and Fe(II)(OH)₄ ²⁻, andthe anode to produce Fe(VI). Optimization testing is needed to determinewhether flow distributors improve or reduce ferrate production rates andefficiencies.

The screens used to separate the anode and cathode should allowelectrolyte and water to pass but retard the mass flow of iron speciesand hydrogen gas from transferring between the anode and cathodecompartments. A membrane is not desirable because of the increasedelectrochemical resistance resulting in much greater power consumption;rather, the screen barrier should be based on macro-scale sizeexclusion, and opposing flow dynamics, to prevent the H₂ gas bubblesfrom migrating to the cathode compartment and an additional catholytestream may be used to further inhibit the reduction of ferrate byhydrogen.

Optimization of the temperature controls to obtain the desiredtemperature of about 50° C. into the cell and about 20° C. throughoutthe remainder of the system will further increase current efficiency andcrystallization. As continuous, or semi-continuous, solids separationoperation is preferred, and an optimized solid/liquid separationoperation is most preferred, for the production of ferrate is preferredover a labor-intensive batch filtration. The tests indicate thatcontinuous solids removal helps to increase current efficiency, inhibitsby-product formation, and/or decomposition of Fe(VI). (see FIG. 17).

A process schematic for one embodiment of the invention is shown in FIG.19. The electrochemical cell, 1900, includes 3 cathodes, 1902, and 2anodes, 1904. Screens, 1906, separate the cathodes, 1902, from theanodes, 1904. The use of screens to separate the cathodic, 1903, andanodic, 1905, compartments may involve the use of a single electrolyte,or separate anolyte and catholyte solutions, with the correspondingpiping and pumps. Using separate electrolytes (anolyte and catholyte)can result, and normally does result, in at least some intermixing ofthe two fluids. The anolyte can be sent to an optional finishing cell,1912, for additional reaction. An in-line, continuous centrifuge orother solids/liquid separation device, 1930, and an optional filter,1935, can be included for continuous separation of solids from theanolyte or combined electrolyte stream. Preferably, the apparatus alsoincludes a means for cooling the anolyte or combined electrolyteentering and/or exiting the cell, heat exchangers, 1916 and 1914,respectively, one or more valves for controlling fluid flow rates, 1920and 1922, in FIG. 19.

Product is recovered as cake of slurry, 1931. Clarified electrolyte ispumped via, 1932, through two-way value through optional polishingfilter, 1935, then through another two-way valve, 1936, through flowcontroller, 1922, through flow meter 1937, then through heat exchanger,1916, then through two-way valve, 1938, to either sample part, 1939 orreturns to the cell 1910 anolyte compartment, 1905. Although FIG. 19only shows electrolyte feed flow to the two anolyte blocks in thediagram, all four actually receive anolyte as cell of the electrodes aresuspended in the centers of their respective compartments. Internalfluid channels separately interconnect all anolyte and catholytecompartments for uniform distribution of electrolyte to all electrodes.

The apparatus of FIG. 19 also includes by-pass hardware consisting ofline, 1941, and valves, 1943 and 1945. This by-pass is used when productisolation is not being performed or when cleaning the apparatus. Theapparatus of FIG. 19 also has the capability to optionally circulate thecatholyte separately from the anolyte. When this feature is used, thecatholyte exits cell, 1910, and flows through a dedicated compartment ofheat exchanger, 1912, to the catholyte surge tank, 1951. The catholytethen is transferred by an air pressure diaphragm pump, 1954, backthrough the heat exchanger, 1956, then sent back to the catholytecompartments, 1903, of the cell, 1910. Variable DC power supply, 1960,provides the adjustable power to cell, 1910, electrodes as required forferrate(VI) production via lead, 1961, to the cathode, 1902, and lead,1963, to the two anodes, 1904.

A preferred embodiment, 2000, of the process of the invention is shownin FIG. 20. An electrolyte is heated to about 40-45° C. using heatexchanger 2001 before entering the electrochemical cell, 2002. Theanolyte leaving the electrochemical cell, 2003 optionally may be sent toa finishing cell, 2005, if desired to increase product yield andstability. Valves 2004 and 2006 control by-pass of the optionalfinishing cell. The anolyte is cooled to a temperature of about 20-25°C. using heat exchanger 2007 and sent to a crystallizer 2009. Thecrystallizer, 2009, may be of any suitable design [Perry's Chemical andEngineering Handbook, Sixth Ed. D. W. Green, Ed., McGraw-Hill pub. (NewYork, N.Y.), 1984, pp 19-77 to 19-85], including a simple tank, a tankwith internal baffling, a tank with internal mixing, a tank withinternal temperature gradients, a tank with internal temperaturegradients controlled by heating elements and/or heat exchangers, a meansfor introducing seed crystals, a tank fitted with external and/orinternal recirculation, etc., and any combination of these, as isobvious to those skilled in the art of crystal growth of solids fromaqueous solutions. The value of control over crystallization is that itallows the product to be produced, and it allows the product to beproduced with controlled morphology (particle shape), and it allows theproduct to be produced with controlled particle size. Althoughmicro-crystalline particles, such as fine needles, are desirable forexample for fast dissolution and battery applications, coarse largecrystals of low surface area or unit weight are most desirable forlowest production for large-scale commodity-priced operations such aswaste water treatment, potable water production and the like. Levelcontrol, 2010, controls valve 2011, which allows pump 2013 to removeslurry from crystallizer 2009, and send it through flow control 2015through sample port valve 2017 at a rate appropriate for centrifuge2019.

After exiting the crystallizer 2009, the anolyte enters a batch,semicontinuous or continuous centrifuge or hydrocyclone, 2019. Theferrate solid cake or slurry is sent to a filter press 2021, to removeadditional electrolyte. Any liquid-solid separation device is sufficientfor product isolation.

Pressurized filtration is preferred though gravity filtration with atleast a slight vacuum is effective. Pressurized filtrations are ofseveral types, either the fluid slurry is pressurized, or the filtercake is pressurized, or both. Most preferred is that both the slurry andthe cake are pressurized. Although high pressures, e.g. 10,000-35,000psig are effective, lower pressures are most preferred, e.g. 1 psig toseveral hundred psig. Many pressurized filtration means are well knownin the art.

The ferrate cake, 2023 can undergo additional processing, such aspelletizing, briquetting, tableting, extrusion, etc. 2025 if desired.The K₂FeO₄ or other ferrate(VI) salt depending on electrolytecomposition. The product is removed at 2027. The electrolyte exits thecentrifuge 2031 and is sent to the electrolyte surge tank 2030 withsimilar and optional recycle electrolyte streams from other points inthe process such as pressure filtrate 2033 and pelletizer liquids 2035.The electrolyte recycle is optional and most preferred.

Electrolyte recycle from the liquid-solid separation device(s) ispreferred since the chemical consumption per unit weight of ferrateproduct is thus reduced and in the amount proportional to the amount offluid recycled. Although it is desirable to discard small amounts oftheses fluids periodically or continuously to purge the process ofaccumulating impurities, such removal is also accomplished by removal ofproduct which simultaneously removes a thin layer of electrolyte on eachproduct particle collected.

Makeup electrolyte, for example NaOH, 2036, and KOH, 2037, can be addedto the surge tank via valve 2040 as needed.

The catholyte exiting the electrochemical cell 2050 is preferably butoptionally sent to a gas/liquid separator 2055 via valve 2057, and thento the surge tank 2030. Valve 2057, or other suitable means, for examplevalves, weirs, etc., to control the flow rate of catholyte 2052 flowrate from cell 2002, serves to minimize the flow of electrolyte to thevalve 2057 and internal divider screens (see FIG. 5 and others) work inconcert to control electrolyte flow across the cathode. Valve 2057performs this control directly by restricting the exiting catholyte flowrate. The internal screen (see FIG. 5 and others) contributes to thisflow control by preventing H₂ gassing agitation from the cathode tocause turbulence in the anolyte compartment. Besides, or in addition to,valve 2057, weirs can be inserted into the exiting anolyte and/orcatholyte flow lines to control the electrolyte flow rate exiting ascatholyte versus that exiting as anolyte.

Contents from the surge tank, 2030, are sent to the cell, 2002 via valve2041, controlled by level control 2042, using pump 2043, through the “bypass line”, 2046, through valve 2047, through flow control valve 2049,through two-way valve 2051 and sample valve 2053, through heat exchanger2001, to cell 2002.

As needed, valves 2045 are switched to transfer at least a portion ofthe electrolyte from the surge tank 2030 from the normal “FilterBy-Pass” condition to a filter 2061 to remove impurities, 2063 ifneeded. For example, ferric hydroxide colloids are removed from theelectrolyte in this manner.

The humid hydrogen gas, 2065, separated by gas separator, 2055, is ofhigh purity and can e released air free as so can be captured as aco-product or vented. During periods where product recovery is notoccurring, for example, during startup or maintenance of solid/liquidseparator three-way valve, 2018, may be opened to allow by-pass ofelectrolyte via the line 2071 to three-way valve 2051. Controlledvariable DC power is provided via power supply, 2073, (see FIG. 2).

The following test procedures are useful for determining the properoperation of the apparatus in the production of ferrate(VI) and of solidferrate(VI) products.

Example 8

The undivided ferrate(VI) production cell apparatus of Example 7 wasoperated further using an in-line filter that was loaded with a spiralwound filter of polypropylene continuous fiber production and rated for10 microns porosity (Serfilco, Ltd. Code No. 15U10U). Other filters ofcorrosion resistant fibers are also acceptable as are other porositiesdue to the self-stacking nature of the microfibers ferrate(VI) productfor uniquely formed by the invention. These micro fibers are new andhave aspect ratios of 5, usually 10 or greater, and most usually 20 orgreater, and normally about 25-35. Crystal lengths can extend to 100microns. Thicker and longer crystals would be prepared using seeding,recirculation and properly placed temperature gradients as is known inthe art. This filter provides continuous solid potassium ferrate(VI)product recovery as described previously.

The solid product was harvested by replacing the filter unitperiodically every one-half, daily, or every other day for the 866 cm²anode area cell previously described operating at 53 A. The thick filterelement spiral wound construction allows substantial loadings of productand prevented filter blinding. In addition, it was determined that thefilter cake also retained a high porosity such that very little pressuredrop occurred across the filter, even when fully loaded with productsolids. Two runs were made of about 22,000 min (15 days), and one run ofmore than 14,000 min (about 10 days). The runs ultimately resulted incomplete consumption of the anode, and hence were taken completely tothe normal maintenance stopping point for such continuous operations. Itis well known to those in the art that for such systems, longer on-linetimes can be designed into the operation by using thicker anodes, largercells, more plates per cell per unit of amperage, etc.

The filled filter cartridges, packed with sodium ferrate(VI) orpotassium ferrate(VI) crystals, (depending upon electrolyte formulation,as described above), are viable ferrate products as they are readilyused by inserting into an in-line filter housing of the same size ormultiple-element sized, and then water circulated through the filterunit. The water dissolves the ferrate(VI) salt and carries it to thepoint of one of the ferrate(VI), for example for surface cleaning, waterpurification, etc. Note that this ferrate(VI) loaded filter units arereadily packaged and stored for later use.

The product was harvested from the filled filter, so that it can be useddirectly or converted to other ferrate(VI) products, using the followingprocedure.

Filter Leach Method for Weight Determination of Ferrate(VI) SaltsAccumulated on Teflon and Spiral Wound Polypropylene CylindricalFilters. Also Useful as a Solid Product Isolation Process.

This method is useful for determinations of the amount of ferrate (VI)solid produced where solid sodium and/or potassium ferrate (VI)crystalline product is removed by filters. An inline filter is used forremoving ferrate(VI) solid from electrolyte flow streams is performed todetermine the amount of ferrate (VI) available for isolation, or byanother solids/liquid separation method. Also, a total iron assay canalso be determined, therewith allowing a total mass and energy balanceto be constructed around the overall electrolytic Fe(VI) productionprocess.

Table 6 shows the centrifuged salt composition as chemical speciesaverages.

TABLE 6 Wt. % Water 43.04% Wt. % NaOH 41.56% Wt. % K₂FeO₄ 7.69% Wt. %Fe“(III)” 4.86% Wt. % CO₃ ⁼ 0.74% Total 97.89%

In this section, the ability of both centrifuges to remove water, sodiumand potassium hydroxides is analyzed. As mentioned herein, batchcentrifugation was tested to see how ferrate(VI) would behave underthese conditions. Two to three liters of electrolyte were removed at onetime, centrifuged at 2500 RPM in a 6×1 L centrifuge for 20 minutes. Thesolids were then collected by decanting the supernatant and then spunsown again in a small 4×50 mL centrifuge for 30 minutes, furtherincreasing the weight percentage if ferrate. There is a plot showing thepercent solids in each of the centrifuged 1 L bottles containingelectrolyte and the concentration of ferrate(VI) in the solids.

Table 6 shows the increase in weight percent ferrate(VI) after thesecond spin down in the 50 mL centrifuge.

Once a filter has been successfully loaded with ferrate (VI) solidproduct, remove it from the system and allow to drain for severalminutes to remove any excess electrolyte, under N₂, or CO₂-free dry airatmosphere. Place any collected liquids back into the surge tank of theprocess as recycled viable electrolyte.

-   -   1. Make up 3-4 L of 6M KOH and cool to about 4.0° C. The final        volume is not important as long as the leaching unit has enough        liquid so the pump does not caveat. Record the exact final        volume of 6M KOH used. For a continuous product isolation        process, the amount of 6M KOH used should be minimized to        minimize costs.    -   2. Load the filter into leaching unit and pass 6M KOH thorough        it for 5 minutes. The KOH solution can be once through, or        preferably recirculated to minimize fluid volumes handled.    -   3. If the objective is to assay for amount of product recovered        by the filter, immediately analyze leachate for Fe(VI) using        procedure described above. Dilution may be necessary to reduce        absorbance values (A505 and A785) to the 0.2-1.2 range. For        product production, the leachate is sent to recrystallization        where for potassium ferrate(VI) product, K₂FeO₄, KOH is added to        about 48-52 Wt % KOH (see enclosed product production        procedure). Sodium, lithium, and blends thereof are similarly        prepared.    -   4. For the purpose of ferrate(VI) production rate analysis, back        calculate the total amount of ferrate(VI) in grams by        multiplying concentration by total leach volume and molecular        weight of ferrate(VI). Some values are given for reference in        Table 5.

TABLE 5 Fe(VI) salt type MW Sodium 165.822 Potassium 198.039 FeO₄ ²⁻only 119.843

Example 9 Determination of Ferrate(VI) Ion Concentration in AqueousSodium and Potassium Hydroxides by UV-VIS Spectrophotometric Analysis

The determination of Fe(VI) as FeO₄ ²⁻ ion is an important quantitativeanalysis for keeping the ferrate(VI) production cells in properoperating condition and at high current efficiencies. It is alsoimportant for determining the active ferrate(VI) content of productfilter cakes and solid products. If the analytical samples are nothandled properly, or the associated UV/VIS spectrum is not interpretedcorrectly, “false high” errors as high as 300% or more in theferrate(VI) production rate are possible. For example, the presence offerrate(III) hydroxide colloids or magnetite crystalline particulates(Fe₃O₄) strongly scatter UV/VIS light in the spectrophotometer, and sowould be erroneously interpreted as being additional ferrate(VI) ifcertain precautions are not taken as described below.

Interference-Free Ferrate(VI) Analysis Procedure:

1. Set up a scanning UV-VIS instrument capable of scanning the region of450 to 850 nm.2. Select the appropriate quartz or glass optical cell. If theconcentration is thought to be higher than 0.01 M, it is recommendedthat a 1.0 mm cell be used. For concentrations below 0.01 M, a 1.00 cmcell is appropriate.3. Check the set of quartz cells, blank and sample, to ensure they matchprecisely and exactly within the range of wavelengths by scanning highpurity deionized (HPDI) water in the cells. Run the baseline for the450-850 nm region with the blank cell holding HPDI water.4. Dilute the electrolyte sample between 3-10 times using 32-34% NaOH.This concentration of NaOH is critical as less results in rapidferrate(VI) decomposition, and more causes problems of incompletedissolution of ferrate(VI). If necessary, check that the ferrate(VI)microcrystalline fibers are dissolved by observing the diluted samplewith an optical microscope. Mix well. The sample should have abubble-free, clear purple color with no visible particles floating insolution. Ensure that the total sample volume is at least 2.5 ml for the1.000 cm cell and 1.0 ml for the 0.100 cm cell.5. Scan the spectrum of the diluted sample at the above specifiedwavelength region and save the recorded spectrum to file. The 785 nmpeak and 505 nm peaks need to have values within 0.100 and 1.200absorbance units. If not, re-dilute another sample with a differentdilution factor that corresponds to higher or lower absorbance valuesdepending on the initial results.6. The spectrum must look like FIG. 24. If peak shape does not matchprecisely, then see troubleshooting section below before proceeding toresolve. Referring now to FIG. 24, this figure is a graph showing theabsorbance as a function of wavelength (λ) of ferrate(VI) spectra. Notethat at “A” there should be a steady decrease in absorbance atwavelengths below the “B” peak of about 505 nm. Note also that at about570 nm there is a small peak integrated with the 505 nm peak. At “D” itis important that this section should not be flat, it should have a niceconcave type shape such as the one shown in FIG. 24. A smaller peak isnoted at about 785 nm. Also it is important that the curve beyond the785 nm peak at about “E” should not be flat.7. Use Beer's Law, A=ε*b*c, to calculate ferrate (VI) ion concentration.Use the 505 nm absorbance value (A505) with ε=1103 M⁻¹ cm⁻¹ as the molarabsorbtivity (ε) value, use cell width in cm for path length (b), andthen concentration (c) will be in molarity (M). Do the same for 785 nmwith a “ε” value of 379 M⁻¹ cm⁻¹. Multiply the result by the dilutionfactor. If the resulting concentrations determined at the twowavelengths do not match to within 0.0005M or 0.5 mM, seetroubleshooting section since a disagreement indicates that contaminantsfrom interferences are present making the determined apparentferrate(VI) ion concentration invalid and erroneously high.

Troubleshooting Guide for Ferrate(VI) Analysis Method

1. Make sure that the sample is mixed well enough, that the quartz celland the detector are clean, and that the sample does not have a lot ofmicro bubbles trapped in solution. If bubbles are present, tap the microbubbles to the top or let settle for one minute before running theanalysis. The cell needs to appear crystal clear when viewed.2. A cause for spectral impurity of the diluted sample may be that theferrate is decomposing during analysis (i.e. there is a rust color incell after analysis). If not sure, set the UV-VIS to real time recordingat single wavelength monitoring at either 785 nm or 505 nm. Re-diluteanother sample with 32-34% NaOH, mix well, and place it into the UV-VISspectrophotometer. If the absorbance values are changing faster than0.003 every minute, it will be necessary to extrapolate the absorbancevalue back to time zero using the single wavelength function (Use the505 nm wavelength for this procedure). Sodium ferrate(VI) unstabilizedsolid cake analyses are most prone to this drifting phenomenon, perhapsdue to the presence of reactive intermediates (see text).

This drift in absorbance value with time can also indicate thatsomething is wrong with the condition of the ferrate production cell,and it should be checked immediately, especially if the A₅₀₅ values arechanging fast (0.001 every few seconds).

3. If ferrate(VI) ion concentration values determined at the twowavelengths still differ after the above steps are taken, then there isiron containing particulate and/or colloidal species contaminating thesample. Centrifuge the diluted sample, for example for three (3) minutesat maximum speed (3000 rpm). Use a transfer pipette to remove the topclear layer of electrolyte, making sure not to shake or otherwise stirthe solids back into solution.4. If none of the previous steps resolve the wavelength discrepancy,filter the diluted electrolyte with a compatible (e.g. polypropylene,polysulfone and other nonreactive filter materials) syringe filter with5 um porosity or less before performing the spectrophotometric analysis.5. If the dual wavelength technique still gives significantly differentapparent ferrate(VI) concentrations, then something is seriously wrongeither with the running condition of the electrolyzer, or theelectrolyte has been contaminated and needs to be replaced. Hence, inthis event, cell shutdown, cleaning and rebuilding is prescribed.

Example 10 Determination of Total Iron Concentration by UV-VIS Analysis

The UV-VIS method for total iron determination is a fast and low costanalysis method. The method was verified by commercial inductivelycoupled argon plasma mass spectrophotometric (ICP-MS) analysis. Withcare, this analysis is accurate to within ±1%.

Procedure

1. Set up a UV-VIS spectrophotometer, capable of a wavelength scan, toscan between 200-500 nm.2. Using a 1.000 cm quartz cell set, test blank and sample cells withHPDI water, to make sure they match exactly and precisely between therange of wavelengths. Test the baseline with the blank cell using HPDIwater.3. Dilute the sample with reagent grade 6 N HCl by a factor of 50-100×.The solution should turn a faint yellow color, and there should be noparticulate floating in the sample. Cap the sample to protect theinstrument from HCl fumes.4. Scan the sample between the specified wavelengths and save the datato disk. The spectrum should look like FIG. 25 except normally withoutthe “UV Lamp Change” feature which is an artifact of the particularspectrophotometer used. This spectrum is that of tetrachloroferrate ion,FeC₄ ⁻.

Referring now to FIG. 25, this figure is a graph of total ironUV/visible absorption spectra with absorbance as a function ofwavelength in nm. It is important to note the UV lamp cross over “A” atabout 300 nm and the filter change “B” at about 380 nm artifact of theinstrument, not part of the spectrum. Note the particular curve shapewith a peak at about 360 nm.

5. Using the 358 nm absorbance value (A358), calculate the concentrationof iron in parts per million (ppm) by dividing the A₃₅₈ by 0.0551 ppm⁻¹and multiplying by the dilution factor. In order for the resulting valueto be precise, the diluted concentration must read between 0.1 and 1.2absorbance units. If the concentration is too high or low, then diluteanother sample with different dilution factor that corresponds to higheror lower absorbance values depending on the initial results.

Example 11 Ferrate(VI) Production Process Flow Diagram

Described below and shown in FIG. 20 is an embodiment showing a plantfor producing ferrate(VI). The unit would be capable of operatingcontinuously, only allowing for anode replacement and maintenance, usingcontinuous solid product removal using any suitable solid-liquidseparation hardware (by one or a combination of hydrocyclone,centrifuge, pressure filtration, “plate and frame” belt press, and thelike filtration in FIG. 20). The invention process improves theefficiencies so that the ratio Fe (total)/Fe(VI) is decreased toward theideal ratio of about 1/1 when operated with continuous orsemi-continuous flow and filtration. Maintaining this parameter lowmaintains efficient ferrate(VI) production in proportion since theproduction of non Fe(VI) iron species are believed to involve Fe(VI)decomposition directly, or involve other oxidation states [e.g. Fe(IV),Fe(V), Fe(II) and Fe(0)] that give rise to Fe(III) products that suspendin the electrolyte as particulates and colloids. Hence preventing thesepost Fe(VI) production reaction provides an excellent means toincreasing ferrate(VI) production yields and production electricalcurrent efficiencies. This topic is discussed in more detail below.

Referring now to FIG. 15, the apparatus shown in this figure can be usedto prepare ferrate(VI) according to the invention.

Bipolar Ferrate(VI) Production Cell

Bipolar electrochemical cells are known in the art to have advantagesover mono polar cells in lower power consumption, and simplicity ofconstruction, especially with respect to electrochemical “cell stacks”,i.e. electrochemical cells containing more than one cell. Althoughnormal cell construction requires power leads to each electrode, inbipolar cells the voltages are applied to each electrode via an electricfield applied across the stack (FIG. 26). Such bipolar cell designs areuseful with the invention. In this manner, dozens and even about 100 to200 electrodes can be so energized using only two power leads, one toeach end plate. Such end-stack power leads may physically be appliedusing one or more wires, but only about two are preferred, still farless than one or two per monopolar electrode.

Referring to FIG. 26, a typical bipolar cell useful with the inventionwill be enclosed by a housing, 2605, having ports as needed. Power issupplied via one or more anode connections, 2610, and one more cathodeconnection, 2620. A plurality of iron containing electrodes, 2630, areplaced in spaced apart relationships, using non-conducting spacersdescribed herein. Optionally, but preferably in one embodiment, ascreen, 2640, is placed between each of the electrodes 2630. The screentypically has an open mesh (e.g. 1 mm holes or smaller) so that liquidflow is not impeded.

For ferrate(VI) production using the invention in a bipolar cell mode,it is preferred to include the screen, 2640, flow modifiers, describedpreviously to prevent FeO₄ ⁼ loss at the cathodes, and most preferred toinclude preferential electrolyte fluid flow path to the anode, providingelectrolyte to the cathode via through-screen flow from the electrolyteentering the anode compartment, and even more preferred, including therestricted catholyte outlet flow rate control design. The electrodes sodeployed can be of the same materials previously listed for themonopolar cell design, i.e. iron, or iron with a plate of nickel on oneside, or of any other iron containing electrode material. Suchferrate(VI) production cells are powered with electronic circuitssimilar to the type already described except that the actual voltageapplied across the cell stack is about the sum of the number of cellstimes Vmax. For example, for 100 plates at 2.7 volts each for Vmax, willrequire a total applied voltage of 270 volts. Such voltages are readilyavailable commercially. Note that while it may be desirable to separatethe heat space gas from above the anodes and cathodes, it is notrequired in that little or no O₂ gas is produced at the anode the cellof the invention, just ferrate(VI) and iron oxide film. Hence most H₂(g) is the only gaseous product and this gas separates quickly into thehead space. Being essentially pure H₂, it can be recovered and put touseful purpose. The ferrate(VI) product is not volatile and exits withthe microcrystals helps further to prevent a reaction between theferrate(VI) and H₂ produced.

While the forms of the invention herein disclosed constitute presentlypreferred embodiments, many others are possible given common knowledgeto those skilled in the arts of chemical processing, electrochemicalcell operation, solids-liquid separation and the like. It is notintended herein to mention all of the possible equivalent forms orramifications of the invention. It is to be understood that the termsused herein are merely descriptive, rather than limiting, and thatvarious changes may be made without departing from the spirit of thescope of the invention.

1. An undivided electrochemical cell comprising: a housing defining anundivided chamber, the housing having one electrolyte inlet and at leasttwo outlets; an anode in the chamber; a cathode in the chamber; and anelectrolyte in the chamber, wherein the anode and the cathode are notgas diffusion electrodes.
 2. The undivided electrochemical cell of claim1 wherein the housing has two electrolyte outlets.
 3. The undividedelectrochemical cell of claim 1 further comprising a fluid flowcontroller in fluid communication with the electrolyte outlets.
 4. Theundivided electrochemical cell of claim 3 wherein the fluid flowcontroller is selected from flow restrictions, valves, screens, fluidflow constrictions, bends or weirs.
 5. The undivided electrochemicalcell of claim 1 wherein the anode is made of a material containing iron.6. The undivided electrochemical cell of claim 1 wherein the anode isselected from solid iron plate, expanded metal mesh, wire mesh, wovenmetal cloth, wire, rod, or combinations thereof.
 7. The undividedelectrochemical cell of claim wherein the anode is selected from iron,steel, “dimensionally stabilized anode” (DSA), titanium, platinum,iridium, and other oxidation resistant electrolytically conductivematerials.
 8. The undivided electrochemical cell of claim 1 wherein aratio of a surface area of the anode to a surface area of the cathode isin the range of 1 to at least about
 10. 9. The undivided electrochemicalcell of claim 1, wherein said cathode has a physically displaced areathat interfaces least 90%, and preferably about 100%, and mostpreferably 110% of the anode area.
 10. The undivided electrochemicalcell of claim 1 wherein the cathode is made of a material selected fromnickel, titanium, platinum, tin, lead, stainless steel, graphite, ironor alloys thereof, or laminates or claddings thereof.
 11. The undividedelectrochemical cell of claim 1 wherein the cathode is selected fromsolid plate, expanded metal mesh, wire mesh, woven metal cloth, wire,rod, or combinations thereof.
 12. The undivided electrochemical cell ofclaim 1 wherein the electrolyte is an alkaline solution of hydroxide.13. The undivided electrochemical cell of claim 12 wherein thehydroxides are selected from NaOH, KOH, or combinations thereof.
 14. Theundivided electrochemical cell of claim 12 wherein the hydroxides areselected from >BioH, KOH, and combinations thereof.
 15. The undividedelectrochemical cell of claim 12 wherein the electrolyte comprises amixture of KOH and NaOH, and wherein a molar concentration of NaOH isgreater than about
 5. 16. The undivided electrochemical cell of claim 15wherein the electrolyte comprises about 40 to about 45 wt % NaOH andabout 0.1 to about 8 wt % KOH.
 17. The undivided electrochemical cell ofclaim 1 further comprising a screen between the anode and the cathode.18. The undivided electrochemical cell of claim 17 wherein the screen ismade of plastic.
 19. The undivided electrochemical cell of claim 17wherein the screen has a mesh size such to occupy at least 25%,preferably at least 50%, and most preferably at least 75% of the areabetween the anode and cathode surfaces, in which the open area consistsof small openings, as is provided by a screen or the equivalent.
 20. Theundivided electrochemical cell of claim 18 wherein the plastic isselected from polyolefins, fluoropolymers, or polyvinyl chloride. 21.The undivided electrochemical cell of claim 1 wherein the housing ismade of a material selected from metal, fiberglass, reinforced plastic(thermoplastic or thermoset) concrete, rubber, or combinations thereof,including constructed as a corrosion resistant liner within rigid-walledstructure.
 22. The undivided electrochemical cell of claim 1 furthercomprising a variable DC power supply operatively connected to theelectrochemical cell.
 23. A method of operating an undividedelectrochemical cell comprising: Providing a housing defining anundivided chamber, the housing having an electrolyte inlet, at least twoelectrolyte outlets, an anode in the chamber, and a cathode in thechamber; introducing an electrolyte into the chamber through theelectrolyte inlet; and controlling an amount of electrolyte flowing outof the electrolyte outlets so that substantially more electrolyte flowspast the anode than the cathode.
 24. The method of claim 23 whereincontrolling the amount of electrolyte flowing out of the electrolyteoutlets comprises providing a valve in fluid communication with theelectrolyte outlets.
 25. The method of claim 23 wherein the electrolyteinlet is located nearer to the anode(s) than the cathode and preferablylocated beneath the anode.
 26. The method of claim 23 whereincontrolling the amount of electrolyte flowing out of the electrolyteoutlets comprises providing weirs in fluid communication with theelectrolyte outlets, the weirs having different heights.
 27. The methodof claim 23 wherein controlling the amount of electrolyte flowing out ofthe electrolyte outlets comprises providing a flow restriction in fluidcommunication with the electrolyte outlets.
 28. The method of claim 23wherein a ratio of the amount of electrolyte flowing past the anode(mL/cm²/sec) to the amount of electrolyte flowing past the cathode(mL/cm²/sec) is at least approximately 60:40.
 29. The method of claim 23wherein a ratio of the amount of electrolyte flowing past the anode tothe amount of electrolyte flowing past the cathode is at least about70:30.
 30. The method of claim 23 wherein a ratio of the amount ofelectrolyte flowing past the anode to the amount of electrolyte flowingpast the cathode is at least 80:20.
 31. The method of claim 23 wherein aratio of the amount of electrolyte flowing past the anode to the amountof electrolyte flowing past the cathode is at least 90:10.
 32. Themethod of claim 23 wherein a ratio of the amount of electrolyte flowingpast the anode to the amount of electrolyte flowing past the cathode isat least 95:5.
 33. The method of claim 20 wherein a ratio of the amountof electrolyte flowing past the anode to the amount of electrolyteflowing past the cathode is at least 99:1.
 34. The method of claim 23wherein the housing has one electrolyte inlet and two electrolyteoutlets.
 35. The method of claim 23 wherein a ratio of a surface area ofthe anode to a surface area of the cathode is at least about 0.8-1.2,including 1.0, preferably about 3 to 6, and most preferably 8-15. 36.The method of claim 23 wherein the electrolyte comprises an aqueousblend NaOH, and wherein the NaOH concentration can vary fromapproximately 17% (5M) to 52% (20M), preferably from about 25-45%, morepreferably from approximately 30-40% (10.0-17.0M).
 37. The method ofclaim 23 wherein the electrolyte comprises an aqueous blend of KOH andNaOH wherein the KOH concentration can range from about 0-15% (0-3.0M),preferable from approximately 0.1-10% (0.018-2.0M), more preferablyapproximately 0.5-8.0% (0.1-1.5 M), and most preferably about 4-8%(0.74-1.5M).
 38. The method of claim 23 wherein the electrolytecomprises an aqueour blend of KOH and NaOH wherein the KOH to NaOH molarratio ranges from about 0.001 to 0.4, preferably from about 0.01 to 025,more preferable from about 0.1 to 0.25, and most preferably from about0.08 to 0.12.
 39. The method of claim 23 wherein the electrolytecomprises about 40 to about 45 wt % NaOH and about 3 to about 6 wt %KOH.
 40. The method of claim 23 further comprising providing a screenbetween the anode and the cathode.
 41. The method according to claim 23,wherein the ferrateVI is continuously harvested.
 42. The method of claim23 further comprising applying a variable direct current between theanode and the cathode, the variable direct current varying between amaximum voltage and a minimum voltage, the minimum voltage being greaterthan
 0. 43. A method for making ferrate(VI) comprising: providing anundivided electrochemical cell comprising an iron-containing anode, acathode, and an electrolyte solution, wherein the electrolyte comprisesan aqueous solution comprising a mixture of KOH and NaOH wherein a molarconcentration of NaOH is greater than about 5 and a molar ratio ofKOH:NaOH is less than 0.1; and applying a voltage between the anode andthe cathode to form the ferrate(VI).
 44. Method wherein the electrolytecompromises an aqueous solution NaOH concentration can vary fromapproximately 17% (5M) to 52% (20M) preferably from approximately 25-45%(8.4-17.0M), more preferably from about 30-45% (10.0-17.0M).
 45. Themethod of claim 43 wherein the electrochemical cell has an electrolyteinlet and at least two electrolyte outlets.
 46. The method of claim 43further comprising controlling an amount of electrolyte solution flowingout of the electrolyte outlets so that substantially more electrolytesolution flows past the anode than the cathode.
 47. The method of claim46 wherein the amount of electrolyte solution flowing out of theelectrolyte outlets is controlled by valves in fluid communication withthe electrolyte outlets.
 48. The method of claim 46 wherein the amountof electrolyte solution flowing out of the electrolyte outlets iscontrolled by weirs in fluid communication with the electrolyte outlets,the weirs having different heights.
 49. The method of claim 46 whereinthe amount of electrolyte solution flowing out of the electrolyteoutlets is controlled by flow restrictions in fluid communication withthe electrolyte outlets.
 50. The method of claim 43 wherein a ratio of asurface area of the anode to a surface area of the cathode is at leastabout
 10. 51. The method of claim 43 wherein the electrolyte solutioncomprises about 40 to about 45 wt % NaOH and about 3 to about 6 wt %KOH.
 52. The method of claim 43 further comprising providing a screenbetween the anode and the cathode.
 53. The method of claim 43 whereinthe voltage is a variable direct current voltage, the variable directcurrent voltage varying between a maximum voltage and a minimum voltage,the minimum voltage being greater than
 0. 54. The method of claim 53wherein the voltage has a frequency of between about 0.01 and about 1000Hz.
 55. The method of claim 53 wherein the voltage produces a currentdensity of between about 4 and about 70 mA.
 56. The method of claim 43further comprising continuously filtering the ferrate(VI) from theelectrolyte solution.
 57. The method of claim 53 further comprisingrecycling the filtered electrolyte solution to the electrochemical cell.58. A method for making ferrate(VI) comprising: providing anelectrochemical cell comprising an iron-containing anode, a cathode, andan electrolyte solution, the electrolyte solution comprising at leastone hydroxide; and applying a variable direct current voltage betweenthe anode and the cathode to form the ferrate(VI), the variable directcurrent voltage varying between a maximum voltage and a minimum voltage,the minimum voltage being greater than
 0. 59. The method of claim 58wherein the minimum voltage is a voltage that substantially overcomespassivation at the anode.
 60. The method of claim 58 wherein the maximumvoltage is a voltage that exceeds a voltage needed to produceferrate(VI).
 61. The method of claim 58 wherein the voltage has afrequency of between about 0.01 and about 1000 Hz.
 62. The method ofclaim 58 wherein the voltage produces a current density of between about4 and about 70 mA.
 63. The method of claim 58 wherein theelectrochemical cell further comprises at least two electrolyte outlets,and further comprising controlling an amount of electrolyte flowing outof the electrolyte outlets so that substantially more electrolyte flowspast the anode than the cathode.
 64. The method of claim 63 whereincontrolling the amount of electrolyte flowing out of the electrolyteoutlets comprises providing valves in fluid communication with theelectrolyte outlets.
 65. The method of claim 63 wherein controlling theamount of electrolyte flowing out of the electrolyte outlets comprisesproviding weirs in fluid communication with the electrolyte outlets, theweirs having different heights.
 66. The method of claim 63 whereincontrolling the amount of electrolyte flowing out of the electrolyteoutlets comprises providing flow restrictions in fluid communicationwith the electrolyte outlets.
 67. The method of claim 58 wherein a ratioof a surface area of the anode to a surface area of the cathode is atleast about
 10. 68. The method of claim 58 wherein the electrolytesolution comprises a mixture of KOH and NaOH, and wherein a molarconcentration of NaOH is greater than about 5 and a molar ratio ofKOH:NaOH is less than about 0.1.
 69. The method of claim 58 wherein theelectrolyte solution comprises about 40 to about 45 wt % NaOH and about3 to about 6 wt % KOH.
 70. The method of claim 58 further comprisingproviding a screen between the anode and the cathode.
 71. The method ofclaim 58 further comprising continuously filtering the ferrate(VI) fromthe electrolyte solution.
 72. The method of claim 71 further comprisingrecycling the filtered electrolyte solution to the electrochemical cell.73. A method for making ferrate(VI) comprising providing a housingdefining an undivided chamber, the housing having an electrolyte inlet,at least two electrolyte outlets, an iron-containing anode in thechamber, and a cathode in the chamber; introducing an electrolytesolution into the chamber through the electrolyte inlet, the electrolytesolution comprising a mixture of KOH and NaOH, wherein a molarconcentration of NaOH is greater than about 5 and a molar ratio ofKOH:NaOH is less than about 0.1; controlling an amount of electrolyteflowing out of the electrolyte outlets so that substantially moreelectrolyte flows past the anode than the cathode; and applying avariable direct current voltage between the anode and the cathode toform the ferrate(VI), the variable direct current voltage varyingbetween a maximum voltage and a minimum voltage, the minimum voltagebeing greater than
 0. 74. The method of claim 73 wherein theelectrochemical cell has two electrolyte outlets.
 75. The method ofclaim 73 wherein the amount of electrolyte solution flowing out of theelectrolyte outlets is controlled by valves in fluid communication withthe electrolyte outlets.
 76. The method of claim 73 wherein the amountof electrolyte solution flowing out of the electrolyte outlets iscontrolled by weirs in fluid communication with the electrolyte outlets,the weirs having different heights.
 77. The method of claim 73 whereinthe amount of electrolyte solution flowing out of the electrolyteoutlets is controlled by flow restrictions in fluid communication withthe electrolyte outlets.
 78. The method of claim 73 wherein a ratio of asurface area of the anode to a surface area of the cathode is at leastabout
 10. 79. The method of claim 73 wherein the electrolyte solutioncomprises about 40 to about 45 wt % NaOH and about 3 to about 6 wt %KOH.
 80. The method of claim 73 further comprising providing a screenbetween the anode and the cathode.
 81. The method of claim 73 whereinthe minimum voltage is a voltage that substantially overcomespassivation at the anode.
 82. The method of claim 73 wherein the maximumvoltage is a voltage which exceeds a voltage needed to produceferrate(VI).
 83. The method of claim 73 wherein the voltage has afrequency of between about 0.01 and about 1000 Hz.
 84. The method ofclaim 73 wherein the voltage produces a current density of between about4 and about 70 mA.
 85. The method of claim 73 further comprisingcontinuously filtering the ferrate(VI) from the electrolyte solution.86. The method of claim 77 further comprising recycling the filteredelectrolyte solution to the electrochemical cell.
 87. A method formalting ferrate(VI) comprising: A. providing a housing defining anundivided chamber, the housing having an electrolyte inlet, at least oneelectrolyte outlets, an iron-containing anode in the chamber, and acathode in the chamber; B. introducing an electrolyte solution into thechamber through the electrolyte inlet, the electrolyte solutioncomprising at least NaOH, wherein a molar concentration of NaOH isgreater than about
 5. C. Flowing electrolyte out the of outlet D.applying a variable DC voltage between the anode and the cathode ofsufficient amplitude to form the ferrate(VI), the variable directcurrent voltage varying between a maximum voltage and a minimum voltage,the minimum applied voltage, being 0 or greater.
 88. The method of claim87, wherein the variable DC voltage is applied to obtain a voltage levelwhere ferrate active film removal exceeds or equals net active filmformation rate for a selected time period, said time period selected tosubstantially prevent excessive film growth.
 89. The method of claim 87wherein the minimum voltage is greater than 0.